THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 

GIFT  OF 

BALDWIN  M.  WOODS 


MILITARY  ALROPLANL5 


AN  EXPLANATORY  CONSIDERATION  OF  THEIR  CHARAC- 
TERISTICS,   PERFORMANCES,    CONSTRUCTION, 
MAINTENANCE  AND  OPERATION,  FOR 
THE  USE  OF  AVIATORS 


GROVER  CjLOENING,  B.  Sc.,  A.  M.,  C.  E. 

Author  of  "Monoplanes  and  Biplanes" 

Formerly  Aeronautical  Engineer  U    S.  Army,  Chairman  Technical  Committee  Aero  Club 
of  America  and  Engineer  with  the  Wright  Co. 

Since  1915  Vice-Pres.  and  Chief  Engineer,  Sturtevant  Aeroplane  Co. 


FOURTH  EDITION 


COPYRIGHT  BY  G.  C.  LOENING 
ALL  RIGHTS  RESERVED 


Printed  by 

W.  S.  Best  Printing  Company 

Boston,  Mass. 

1917 


Engineering 
Ubrary 

TL 


17 


TABLE   OF   CONTENTS 


CHAPTER  I  Introduction : 9 

CHAPTER  II  Types  of  Aeroplanes 13 

CHAPTER  III  Primarily  for  Reference 25 

CHAPTER  IV  Air  Resistances 41 

CHAPTER  V  Inclined  Surfaces 57 

CHAPTER  VI  Aerodynamic  Theory 69 

CHAPTER  VII  Characteristics  of  Aerofoils 73 

CHAPTER  VIII  Characteristics  of  the  Aeroplane 89 

CHAPTER  I X  Stresses  and  Safety  Factors 105 

CHAPTER   X  Assembly  and  Construction 121 

CHAPTER   XI  Marine  Aeroplanes 139 

CHAPTER   XII  Flying,  Stability  and  Airworthiness 147 

CHAPTER   XIII  The  Eyes  of  the  Army  and  Navy 171 

CHAPTER   XIV  Conclusion..  175 


PREFACE  TO  SECOND,  THIRD  AND  FOURTH  EDITIONS 

Although  great  strides  have  been  made  in  the  application  of  military 
aeroplanes  to  problems  of  strategy  and  tactics,  actual  war  lessons  show 
that  the  principles  of  design,  construction,  and  flying  remain  the  same. 
Many  important  improvements  in  details,  however,  are  given  great  im- 
petus by  the  exacting  and  inspiring  rivalry  of  war. 

The  object  of  this  book,  in  assisting  military  aviators  to  acquire  a 
more  intimate  knowledge  of  their  machines,  appears  to  be  attained,  in 
that  at  the  military  and  naval  aviation  schools  of  several  nations  it  has 
been  adopted. 

These  new  editions  are  corrected,  more  conveniently  re-arranged  and 
somewhat  enlarged. 

Boston,  June,  1917. 


PREFACE 

That  military  or  naval  aviators  should  desire  to  acquire  a  sound 
knowledge  and  just  appreciation  of  the  machines  to  which,  day  after 
day,  they  entrust  their  lives,  is  but  natural.  And  at  the  suggestion 
of  the  officers  of  the  Signal  Corps  Aviation  Section,  the  writer  has 
gathered  together  some  information  acquired  in  practical  experience, 
into  the  form  of  a  text-book  for  flyers. 

Based,  in  its  composition,  on  questions  asked  and  information 
sought  by  military  aviators,  and  written  practically  on  the  field,  at 
the  largest  aviation  center  in  this  country,  with  unusual  facilities  for 
inspection,  test,  flying  and  discussion  of  aeroplanes  —  every  effort 
has  been  made  in  this  work  to  permit  this  practical  atmosphere  to 
permeate  its  pages. 

It  is  to  be  noted  that  enlargement  on  or  repetition  of  any  matter 
contained  in  the  author's  previous  work,  "Monoplanes  and  Biplanes," 
has  been  avoided.  There  is  presented  here  a  new  text-book,  limited 
to  the  practical  consideration  of  Military  Aeroplanes  in  a  manner 
particularly  applicable  on  an  aviation  field,  and  containing  knowledge 
that  every  aviator  should  have. 

Occasion  is  taken  to  point  out  that  the  considerations  of  flying, 
stability,  airworthiness  and  performances,  are  based  on  experiences 
of  the  author  himself,  in  acting  as  observer,  noting  aeroplane  move- 
ments, reading  instruments  and  taking  observations,  in  flight  (a  specialty 
to  which  the  writer  has  devoted  scores  of  hours  in  the  air),  particularly 
in  the  extensive  experimental  flying  on  the  Signal  Corps  aeroplanes 
designed  by  him  and  piloted  by  Lieut.  T.  DeWitt  Milling.  To  the 
latter,  the  author  wishes  to  express  appreciation  of  much  valuable 
co-operation  and  assistance;  and  he  is  also  indebted  to  Capt.  Town- 
send  F.  Dodd,  Lieut.  Walter  R.  Taliaferro,  George  Hallett,  and  Oscar 
A.  Brindley,  expert  aviator,  for  many  valuable  suggestions  and  assist- 
ance in  proof  reading,  and  to  Capt.  Arthur  S.  Cowan,  in  command,  for 
every  encouragement  in  this  work. 

Opportunity  cannot  too  often  be  taken  by  aeronautical  engineers 
to  recognize  and  pay  tribute  to  the  great  work  of  the  Aerodynamical 
Laboratories,  and  in  particular  to  the  labors  of  the  eminent  French 
engineer,  Gustav  Eiffel,  whose  exhaustive  tests  and  their  splendid 
presentation  form  a  basis  for  accurately  predicting  performances  that 
one  cannot  help  but  marvel  at.  This  work,  and  the  reports  of  the 
British  Advisory  Committee,  have  been  freely  consulted,  and  refer- 
ence frequently  made  to  information  they  contain. 

Coronado,  Cal.,  May,  1915. 


MILITARY  AEROPLANES 


CHAPTER    I. 


INTRODUCTION 

Although  Aviation  is  a  new  field  of  human  endeavor,  its  appli- 
cation to  the  art  of  warfare  is  already  becoming  a  specialty.  Only 
recently  has  it  been  appreciated,  that  military  requirements  have  a 
most  vital  and  important  influence  on  many  features  of  aeroplanes  — 
not  only  in  the  art  of  using  them  in  military  operations,  but  in  their 
fundamental  design  and  construction. 

It  is  planned,  therefore,  to  give  particular  attention  here  to  the 
military  aeroplane,  as  we  find  it  today  —  emerged  from  a  crude  state 
of  invention  and  development  into  a  more  or  less  finished  product, 
which,  in  the  greatest  war  of  history,  has  gloriously  demonstrated  its 
strategical  and  tactical  importance. 

It  is  no  longer  necessary  to  speculate  on  the  uses  of  aeroplanes  in 
warfare.  What  has  actually  been  accomplished  in  directing  artillery 
fire,  in  reconnaissance,  in  dispatch-carrying,  and  in  offensive  work 
has  opened  a  new  phase  of  warfare,  as  significant  as  it  is  surprising. 

The  technique  of  the  use  of  aeroplanes  in  strategy  and  tactics,  is 
decidedly  a  subject  for  the  military  expert,  but  the  general  design 
and  construction  of  aeroplanes  to  accomplish  certain  definite  purposes, 
and  their  operation  and  maintenance  in  the  field,  are  subjects  that  may 
properly  be  considered  here. 

In  addition  to  expert  ability  in  their  operation,  it  is  found  that 
a  sound  and  practical  knowledge  of  the  design  and  construction  of 
aeroplanes  is  exceedingly  helpful  to  the  military  aviator. 

A  full  consideration,  therefore,  is  given  to  elementary  theory  and 
practice  applied  in  aviation,  and  the  information  used  is  primarily 
designed  to  be  of  definite  service,  in  the  field,  where  many  unforeseen 
difficulties  constantly  arise. 


Before  taking  up  the  determination  of  its  elements,  it  is  neces- 
sary, clearly,  to  distinguish  the  aeroplane  from  other  craft  designed 
to  navigate  the  air. 

Aircraft  may  be  divided  into  the  following  classes : 


10 
I.     AIRSHIPS  OR  DIRIGIBLE   BALLOONS. 

The  "airship,"  is  distinctly  a  lighter-than-air  machine,  con- 
sisting of  a  balloon  or  gasbag,  containing  a  gas  —  hydrogen  for 
example  —  lighter  than  air,  which  by  displacement  of  an  equal 
volume  of  air,  gives  a  notation,  the  magnitude  of  which  is  de- 
termined by  the  kind  of  gas,  the  size  of  gas  container,  and  atmos- 
pheric conditions.  The  ordinary  free  balloon  is,  in  short,  nothing 
more  than  a  harnessed  "bubble,"  and  the  dirigible,  or  airship, 
is  a  balloon  of  elongated  shape,  fitted  with  steering  apparatus 
and  propelling  mechanism. 

Airships  are  constructed  mainly  in  three  different  types, 
the  "Rigid,"  the  "Semi-Rigid"  and  the  "Flexible  or  Non-Rigid." 
These  designations  refer,  entirely,  to  the  manner  of  combina- 
tion of  gas  container  and  framework  carrying  the  weights  of  en- 
gines, etc.  A  flexible  gas  container,  held  in  shape  only  by  the 
pressure  of  gas  within  and  to  which  the  load  is  hung,  character- 
izes the  "Non-Rigid"  system.  A  gas  container,  held  in  shape  by 
gas  pressure,  with  an  additional  stiffening  keel  to  which  the  weights 
are  attached,  is  descriptive  of  the  "Semi-Rigid."  Whereas,  in  the 
"Rigid"  system,  a  stiff,  braced  frame-work  or  hull,  carrying  di- 
rectly the  motors  and  loads,  is  formed  to  contain  within  it  numer- 
ous separate,  drum-shaped  gas  containers  instead  of  balloons. 
The  stiff  frame  provides,  in  itself,  that  necessary  rigidity  of  hull, 
which  interior  gas  pressure  on  the  envelope  provides  in  the  other 
types. 


The  Zeppelin  airship  was  the  first  successful  development  of 
the  rigid  system. 


A  Zeppelin  "Rigid"  airship  and  above  it  an  aeroplane.     The  airship  can  float  at  rest 
but  an  aeroplane  must  acquire  speed  in  order  to  fly. 


11 

2.  FLYING  MACHINES. 

The  Aeroplane  —  In  distinction  to  the  airship,  supported  in  the  air 
by  a  buoyant  gas,  the  aeroplane  is  supported  by  an  upward  wind 
pressure,  generated  by  its  own  speed  through  the  air.  This  lift- 
ing pressure  is  obtained  on  specially  formed  wing  surfaces,  which 
are  set  at  an  inclined  angle,  and  forced  through  the  air  at  the  re- 
quired speed  by  an  air  propeller.  Suitable  auxiliary  surfaces 
and  rudders  are  used  to  preserve  the  equilibrium  of  the  craft  and 
to  enable  the  pilot  to  steer  it. 


The  Helicopter  —  Air  propellers  are  similar  in  character  to  marine 
screw  propellers,  and  not  only  are  they  made  use  of  to  push  or 
pull  an  aeroplane,  but  it  has  been  proposed,  in  operating  them 
on  a  vertical  axis,  to  use  their  thrust  directly,  in  lifting  loads. 
This  type  of  flying  machine  is  called  the  "Helicopter"  or  "direct 
lift"  machine,  and  does  not  involve  the  principle  of  lift  from  the 
inclined  arched  plane,  used  in  the  aeroplane. 


The  Ornithopter  —  Nature's  flying  machines  —  the  birds  —  are  neither 
screw  propelled  aeroplanes  nor  helicopters.  They  derive  their 
support  from  the  wind  pressure  on  their  outstretched  wings  pre- 
cisely as  does  the  aeroplane,  but  for  propulsion,  the  bird  flaps 
its  wings  in  a  rowing,  weaving  motion,  which  gives  a  forward 
push.  When  an  aeroplane  glides,  it  resembles  in  character  the 
soaring  of  a  bird,  with  wings  outstretched,  but  attempts  to  de- 
rive propulsion  from  a  reciprocating  movement  of  wings,  have 
not  been  successful,  as  yet.  Machines  of  this  type  are  called 
"Ornithopters"  or  "Flapping-wing"  Machines. 


Although  little  has  been  accomplished  with  them,  the  possibil- 
ities of  the  helicopter  and  ornithopter  have  by  no  means  been  fully 
investigated,  and  whether  or  not  a  combination  of  "direct  lift"  and 
aeroplane,  often  called  the  "gyroplane,"  has  any  future,  is  still  a  sub- 
ject for  study. 

Airships,  on  the  other  hand,  are  very  highly  developed,  and  al- 
though they  are  difficult  to  handle  and  very  expensive,  they  are  looked 
upon  as  "battleships"  of  the  air.  Their  design  and  construction  are 
full  of  interesting,  and  difficult,  engineering  problems,  and  it  is  planned 
to  give  them  consideration  elsewhere. 

In  this  connection  it  is  important  to  point  out,  that  the  oft-stated 
"principle,"  that  aeroplanes  are  limited  in  size,  due  to  a  proportionally 
greater  increase  in  weight  as  the  size  is  increased,  is  a  fallacy,  and,  as 
a  matter  of  fact,  recent  work  on  large-sized  machines,  appears  to  demon- 


12 

strata,  that  in  proportion  to  the  weight  of  the  machine,  as  the  size 
increases,  a  greater  excess  load  can  be  carried.  (In  later  chapters  this 
feature  will  be  further  investigated.)  Aeroplane  "battleships"  are,  by 
no  means,  an  impossibility.  The  consideration  of  large-size  aircraft, 
therefore,  becomes  merely  an  efficiency  comparison  of  the  lift  by  gas 
bag  and  the  lift  by  air  pressure  on  planes.  If  the  dirigible  balloon  lifts 
more  "live  load,"  per  pound  head  resistance,  at  the  same  speed  than  does 
an  aeroplane,  the  dirigible  is  apt  to  survive. 

Of  the  various  kinds  of  aircraft,  only  one  type  of  flying  machine 
is  to  be  considered  here,  primarily,  because  we  find  the  aeroplane,  at 
present,  the  most  successful,  the  most  economical  and  the  best  developed 
means  of  navigating  the  air. 


CHAPTER   II. 
TYPES   OF  AEROPLANES 

At  the  present  time  the  early  inventive  stage  in  the  development 
of  the  aeroplane  is  gradually  but  perceptibly  giving  way  to  a  state  of 
more  precise  engineering.  And,  in  this  step  in  its  progress,  aviation 
is  but  following  the  course  taken  by  almost  every  other  art  and  sci- 
ence. Any  classification  of  aeroplanes,  therefore,  is  subject  to  modi- 
fication as  newer  craft  are  developed,  and  old  ones  rendered  obsolete. 
But  the  general  principles  of  the  machines  do  not  change  as  rapidly  as 
do  their  concrete  interpretations. 

The  principle  of  sustentation  of  an  aeroplane  from  the  upward 
push  of  air  flowing  past  it,  has  been  stated,  and,  in  the  following  chap- 
ters, will  be  analyzed.  The  support  being  derived  from  the  free  air, 
an  aeroplane  is  readily  subject  to  loss  of  balance,  due  to  air  disturb- 
ances, gusts,  convection  currents,  etc.  It  follows,  therefore,  that 
many  features  designed  to  overcome  loss  of  balance,  are  used  on  aero- 
planes. Organs  are  also  introduced  to  give  the  pilot  control  over  the 
craft  within  definite  limits. 

An  aeroplane  consists,  therefore,  of  lift-generating  surfaces  at- 
tached to  a  frame  carrying  motor,  fuel,  pilot  and  equipment,  and  in 
combination  with  devices  to  balance  and  steer  the  craft. 

Flying  freely,  in  the  air,  an  aeroplane  has  three  axes  of  rotation. 

1.  It  may  ascend  or  descend,  by  virtue  of  changes  in  its  longi- 
tudinal path.     The  nosing  up  and  nosing  down  of  an  aeroplane  is  termed 
"pitching,"  as  in  boats. 

2.  An  aeroplane,  in  flight,  may  change  its  direction  of  travel. 
This  is  termed  "yawing,"  as  in  boats. 

3.  In  addition  to  these,  an  aeroplane  can  tip  over  to  either  side, 
on  a  transverse  axis,  and  this  movement  is  termed  "banking"  or  "roll- 
ing."    In  making  turns,  it  is  necessary  to  "bank"  up  an  aeroplane, 
side  wise,  sufficiently  to  overcome  the  centrifugal  force,  and  prevent 
skidding.     This   "banking"   is   obtained   by   manipulating  the  lateral 
control. 

The  locomotive  driver,  is  steered  by  the  tracks,  and  has  to  give 
his  attention,  only  to  the  control  of  the  speed  of  his  engine;  an  auto- 
mobile driver,  controls  his  motor,  also,  but  in  addition  must  steer  his 
machine;  whereas  the  aeroplane  pilot  both  steers  and  operates  his 
engine,  and  in  addition  must  give  his  best  attention,  continually,  to 
balancing  the  machine,  fore  and  aft  and  side  to  side. 


14 

Like  every  science,  Aviation  has  a  language  of  its  own,  and  a 
method  is  used  here  of  expressing  this  language  in  photographs.  Study 
of  the  explanatory  caption  and  of  the  photographs  themselves,  there- 
fore, is  equal  in  importance  to  the  reading  of  the  text. 

The  types  of  aeroplanes  considered  here  are  typical  ones  of  dis- 
tinct features,  and  a  more  detailed  discussion  of  their  merits  will  be 
found  in  later  chapters. 

THE  "TRACTOR"  AND  THE  "PUSHER" 

An  aeroplane,  that  is  pulled  through  the  air  by  a  propeller  situ- 
ated at  the  front  of  the  machine,  is  called  a  "tractor." 

On  the  other  hand  if  the  propeller  is  back  of  the  main  lifting  planes, 
the  machine  is  called  a  "pusher."  These  terms  are  very  expressive 
and  very  widely  used. 

The  single  propeller  "tractor"  is  the  most  widely  used  type  now, 
but  the  "pusher"  type,  particularly  for  gun-carrying,  has  still  a  "raison- 
d'etre." 

The  term  "biplane"  refers  to  an  aeroplane  with  wings,  super- 
imposed, and  "monoplane"  to  a  single  deck  type  of  plane. 

THE  CONTROLS. 

Since  there  are  three  axes  about  which  an  aeroplane  may  rotate, 
it  follows  that  three  controlling  organs  are  required : 

1.  The  "elevator,"  for  pitching; 

2.  The  "rudder,"  for  steering  or  "yawing;" 

3.  The  "lateral"  or  "rolling"  control. 

The  principle  of  the  air  force  derived  from  an  inclined  plane,  is 
used  in  all  of  these  controls.  The  "elevator"  is  inclined  up  or  down, 
to  lift  or  depress  the  tail  of  the  machine.  The  rudder  is  turned  so 
as  to  permit  the  wind  to  blow  on  it,  to  one  side  or  the  other,  whereas 
the  lateral  control  consists,  merely,  in  giving  a  difference  in  angle  to 
the  two  sides  of  the  wings,  causing  one  side  to  lift  more  than  the  other. 

There  are  three  general  means  of  lateral  control : 

1.  "Ailerons,"  or  separate  small  planes,  on  either  side  independ- 
ent of  the  main  lifting  surface; 

2.  "Wing  flaps,"  or  portions  cut  out  of  the  main  surface  and 
hinged  thereto; 

3.  "Warping,"  which  consists  in  twisting  the  main  lifting  sur- 
face, so  as  to  get  a  greater  angle  of  inclination  to  the  wind  on  one  side 
and  less  on  the  other. 

In  the  construction  of  rudders  and  elevators,  the  necessary  change 
in  angle  to  alter  the  wind  pressure,  is  accomplished  either  by  pivoting 
the  entire  surface,  or  by  turning  a  flap  hinged  to  a  fixed  surface  in  front 
of  it. 


15 


Above  —  Rear  view  of  tractor,  with  overhang  wings,  and  wing  flaps  for  lateral  control. 
Center  —  Front  view  of  tractor  with  ailerons. 

Below  —  Rear  view  of  tractor  with  equal  planes,  and  lateral  control  flaps  on  both  upper 
and  lower  planes. 


The  combination  of  fixed  tail  plane  and  movable  flap  is  often  termed  a  "flap  and  fin" 
elevator. 


16 
THE  TRACTOR  BIPLANE. 

The  form  of  aeroplane  that  at  present  approaches  the  nearest 
to  a  standardized  type  is  the  Tractor  Biplane. 

The  main  lifting  surface,  as  may  be  seen  from  the  photographs, 
consists  of  two  super-imposed  planes,  with  their  widest  dimension 
across  the  flight  path. 

The  main  planes  are  attached  to  a  long,  fish-shaped  body,  termed 
the  "fuselage,"  which,  in  effect,  is  the  backbone  of  the  machine,  since 
it  carries  the  motor  and  propeller  at  the  front  and  the  seats  near  the 
center,  while  at  the  extreme  rear  are  mounted  the  rudder  and  elevator. 

The  use  of  an  enclosed  fuselage  in  a  tractor  type  is  almost  uni- 
versal, and  greatly  increases  the  efficiency  of  a  machine,  by  reduction 
of  head  resistance  in  the  wind.  The  disposition  of  the  seats  in  the 
body  gives  excellent  protection  to  the  aviators.  It  will  be  noticed 
that  two  seating  arrangements  are  shown  —  "tandem,"  one  ahead  of 
the  other,  and  "side  by  side."  The  former  is  good  for  military  scouting, 
and  the  latter  possibly  for  training. 

In  the  types  of  tractor  biplanes  shown,  the  chassis  is  mounted 
to  the  body,  as  is  also  the  center  section  of  the  wings.  By  taking  the 
outer  wings  off,  this  type  is  readily  made  transportable  by  road. 

The  distinction  between  a  double  flap  and  single  flap  elevator  is 
shown  in  the  illustrations,  and  there  is  also  shown  the  difference  between 
ailerons  and  flaps  for  lateral  control. 

In  the  photograph  of  the  biplane  tractors  in  flight,  several  de- 
tails show  up  clearly, ' —  particularly,  angles  of  view  of  the  pilot,  whose 
vision  is  interfered  with  by  the  lower  plane. 

PUSHER  BIPLANES. 

The  older  types  of  machines,  particularly  the  early  Wright  and 
Curtiss,  were  pusher  types  —  the  Wright,  however,  had  two  propellers 
and  the  Curtiss  only  one.  These  types  were  open-bodied,  entirely 
unprotected,  and  with  the  motor  to  the  side  of  or  behind  the  aviator. 

A  few  years  of  development,  led  to  the  adoption  of  either  a  na- 
celle —  short  fuselage,  protecting  seats  and  motor  only,  —  or  a  fuselage. 
In  using  a  fuselage  on  a  "pusher"  machine,  it  becomes  necessary  either 
to  mount  a  propeller  at  the  extreme  rear  "torpedo"  fashion,  to  mount 
a  propeller  on  either  side,  or  to  have  a  propeller  running  on  a  large  bear- 
ing around  the  fuselage.  In  "pusher"  flying  boats  the  propeller  tips 
just  clear  the  boat. 

The  earliest  Wright  machine  had  the  elevator  in  front,  so  that 
to  ascend  the  elevator  was  turned  up,  thus  lifting  up  the  nose,  and 
vice  versa;  whereas,  when  it  was  later  changed  to  the  rear,  for  reasons 
of  stability,  to  ascend  it  became  necessary  to  turn  the  elevator  the 
opposite  way,  thereby  pressing  down  the  tail.  This  distinguishes  "front 
elevator"  and  "rear  elevator." 


17 


MILITARY   TRACTORS 

Above  —  Tractor  with  stagger,  overhang,  wing  flaps  and  flat  span. 

Center  —  Tractor  with  ailerons  and  dihedral  span.     The  rudder  has  no  fixed  surface 

in  front  of  it,  and  being  hinged  so  as  to  balance  the  air  pressures,  it  is  called  a 

"balanced"  rudder. 
Below  —  Tractor  with  double  flaps,  high  rudder  and  fins  for  directional  stability. 


IS 


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"PUSHER"    BIPLANES 

Above —Left —Wilbur  Wright,  the  inventor,  and  the  early  type  of  Wright  double  pusher 
biplane,  with  elevator  out  in  front.  Right — Double  screw  pusher  Wright  biplane, 
of  later  pattern,  elevator  in  rear. 

Center  —  Twin  screw,  pusher  fuselage  biplane,  with  engine  in  front. 

Bottom  —  Left  —  Early  Curtiss  open  body,  pusher  —  one  screw,  three  wheel  chassis, 
rudders  in  rear.  Right  —  Farman  pusher  biplane  with  nacelle  or  enclosed  body. 


A  "fuselage"  encloses  motor  seats,  etc.,  but  in  addition  serves  as  the  main  structural 
unit  of  a  machine,  whereas  a  "nacelle"  serves  merely  for  wind  protection,  since  a 
separate  frame  carries  the  rudders. 

The  term  "empennages"  refers  to  the  tail  surfaces  of  a  machine,  whether  they  be  "bal- 
anced" or  "flap  and  fin." 

The  term  "fin"  largely  replaces  the  term  "keel."  It  will  be  noted  that  the  early  Wright 
machines  have  no  fins  or  keels  in  the  empennages. 

The  side  surfaces  of  an  enclosed  fuselage  are  virtually  keels. 


20 


21 
MONOPLANES. 

It  has  often  been  the  custom,  distinctly  to  separate  biplanes 
and  monoplanes,  as  different  types.  This  is  hardly  justified,  since  the 
only  distinguishing  feature  is  the  use  of  a  single  deck,  "king  post" 
type  of  truss  to  carry  the  air  pressure  lifting  load,  in  the  monoplane, 
and  a  double  deck,  "Pratt"  type  truss,  in  the  biplane.  Biplane  sur- 
faces, do  interfere  slightly  with  each  other,  but  in  tractors  the  disposi- 
tion of  motor,  wings,  body,  rudders  and  even  chassis,  is  identical,  whether 
biplane  or  monoplane. 

A  further  misconception,  in  this  connection,  is  that  the  monoplane 
is  faster  than  the  biplane.  The  more  recent  speed  scout  biplanes  have 
proved  the  fallacy  of  this,  and,  in  later  chapters,  it  will  be  found  that 
biplane  and  monoplane  are  both  similar  aeroplanes,  differing  primarily 
in  wing  surface  bracing. 

Several  monoplane  photographs  are  given  on  the  opposite  page. 

Monoplanes,  like  biplanes,  may  be  tractors,  pushers,  open-bodied, 
or  have  two  propellers.  Several  European  firms  construct  a  body 
and  chassis,  complete  with  rudders,  to  which  either  monoplane  or  bi- 
plane wings  may  be  mounted. 

In  general,  the  biplane  carries  more  load,  and  the  monoplane  is 
simpler  in  construction.  But  even  these  differences  are  fast  disap- 
pearing. 

A  distinct  advantage  of  the  tractor  monoplane  over  the  tractor 
biplane,  is  found  when  the  wings  of  the  monoplane  are  raised  slightly 
above  the  body,  thereby  enabling  the  pilot  to  look  under  them  and 
to  have  a  free  and  unobstructed  view. 

AEROBOATS  OR  FLYING   BOATS. 

For  the  purpose  of  starting  from  and  alighting  on  water,  aero- 
planes of  tractor,  pusher,  or  any  type  are  readily  modified. 

Merely  adding  pontoons  to  a  tractor,  in  place  of  wheels,  gives 
the  hydro-aeroplane;  and  the  construction  of  aeroplanes,  fitted  to 
receive  either  wheels  or  pontoons,  as  circumstances  require,  has  de- 
veloped considerably.  Craft  of  this  kind  are  called  "convertibles." 

But  in  order  to  obtain  greater  sea- worthiness  and  better  co-ordi- 
nation in  design,  a  special  type  of  aeroplane  has  been  developed,  suit- 
able only  for  over-water  work.  The  keynote  in  its  design  is  found 
in  its  treatment  as  a  boat  with  wings,  rather  than  an  aeroplane  with 
floats.  The  aeroboat,  or  flying  boat,  therefore,  is  primarily  charac- 
terized by  a  staunch,  boat-like  body,  around  which  the  rest  of  the 
aeroplane  is  built.  The  photographs  show  several  different  types. 

For  further  discussion  of  aeroboats  and  hydro-aeroplanes  refer- 
ence is  made  to  the  chapter  specially  devoted  thereto. 


22 


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23 
THE  "DUNNE"  AEROPLANE. 

In  the  preceding  types,  the  auxiliary  organs  for  pitching  and  yaw- 
ing are  separated  from  the  main  planes  and  are  distinct.  In  the  Dunne 
aeroplane,  there  is  only  one  set  of  controlling  organs,  and  due  to  the 
peculiar  shape  and  construction  of  the  machine,  the  control  of  yaw- 
ing, pitching  and  rolling  is  combined  and  governed,  only,  by  the  double 
wing  flaps.  As  may  be  seen  from  the  illustrations,  the  main  planes 
are  set  in  a  "retreating"  position.  Their  position  in  plan,  and  their 
angle  setting,  give  inherent  stability  characteristics,  which  will  be  taken 
up  irf  a  later  chapter. 

The  "Dunne"  principle  of  a  retreating  plane  is  used,  though  in 
a  modified  way,  in  the  German  aeroplanes,  called  "Pfeilfliegers"  or 
"Arrowplanes,"  but  the  customary  fuselage  and  rudders  are  retained. 
The  German  "Taubes"  are  monoplanes  with  pigeon-like  retreating 
wings.  (See  p.  170.) 

It  may  be  stated  here,  that  the  "retreating"  planes  have  much 
the  same  effect  as  a  dihedral  angle,  on  lateral  stability,  but  are  not 
so  sensitive  to  side  puffs.  The  effect  on  pitching  stability,  obtained 
on  the  Dunne,  by  the  negative  incidence  at  the  tips,  can  be  had,  though 
in  a  lesser  degree,  on  the  more  ordinary  types  of  aeroplanes,  by  a  nega- 
tive setting  of  the  tail  planes.  While  "inherent"  stability  is  descrip- 
tive of  that  obtained  by  the  construction  of  the  aeroplane  itself,  in  shape, 


THE  U.  S.  ARMY  DUNNE  TYPE  BIPLANE 

The  changing  wing  section  and  reducing  angle  of  incidence  are  clearly  seen. 

The  bustle  is  used  to  deflect  the  air  sideways.  The  wing  flaps  on  upper  and  lower 
planes,  are  the  only  means  of  control.  To  ascend  all  flaps  are  turned  up,  and  to  de- 
scend they  are  all  turned  down.  Inverse  movement  rolls  the  machine  laterally,  causing 
it  to  turn. 


24 

wing  setting,  balance  and  fin  disposition,  a  clear  distinction  is  drawn 
between  stability  of  this  type  and  that  obtained  by  adding  to  any  aero- 
plane an  auxiliary  mechanism,  designed  to  be  actuated  by  movements 
of  the  aeroplane,  and  automatically  operating  the  controls,  for  proper 
corrective  effect.  Such  a  mechanism  is  virtually  an  automatic  pilot, 
and  is  often  termed,  a  stabilizer.  "Automatic"  stability  may  be  ob- 
tained, by  use  of  a  mechanism  of  this  nature,  on  an  aeroplane  that  is 
inherently  lacking  in  stability. 

There  are  many  other  types  of  aeroplanes,  but  their  general  features 
resemble  those  described,  and  the  art  moves  too  quickly  to  give 'them 
all  consideration.  A  general  idea,  of  the  various  types,  having  been 
given,  a  more  detailed  study  of  the  aeroplane  may  be  taken  up. 


STURTEVANT  MILITARY  TRACTOR 
A  load-lifting  type,  built  almost  entirely  of  steel  construction. 


CHAPTER   III. 
PRIMARILY  FOR  REFERENCE 

As  much  as  possible,  mathematics  are  avoided  in  the  technical 
parts  of  this  work.  Where  formulae  are  of  real  help,  however,  in  stat- 
ing clearly  the  relation  between  quantities,  they  are  used  and  fully 
explained. 

In  a  field  like  this  one,  so  eminently  practical  in  its  nature,  com- 
mon-sense is  of  much  greater  benefit  than  abstruse  scientific  knowl- 
edge. There  is  justification  for  decrying  the  vast  amount  of  com- 
plicated mathematics  that  have  been  built  up  on  fundamental  assump- 
tions which  the  practical  air  pilot  knows  are  wholly  erroneous,  but 
in  doing  so,  let  us  not  forget  that  scientists  and  the  laboratories  have 
contributed  a  great  and  valuable  share,  in  advancing  the  aeroplane's 
efficiency. 

It  is  praiseworthy  in  presenting  a  subject,  to  simplify  it,  and  to 
avoid  a  too  technical  impression,  but  where  this  is  at  the  expense  of 
a  clear  and  full  understanding,  it  is  inadvisable. 

Aeroplanes,  as  machines,  naturally  involve  many  scientific  ele- 
ments, and  it  is  certainly  best,  at  the  outset,  to  realize  this  and  to  ac- 
quire a  working  conception  of  what  they  are. 

1 .  It  is  necessary  to  know  the  simpler  types  of  equations  and  why 
they  are  so  handy. 

2.  The  elements  used  in  solving  triangles,  such  as  sines  and  cosines 
of  angles,  should  be  familiarized,  and  a  logarithm  table  is  sometimes 
very  convenient. 

3.  Mechanics    dealing    with    momentum,    inertia,    accelerations, 
centrifugal  force,  and  gyroscopic  force,  should,  at  least,  be  understood, 
and  a  comprehensive  review  should  be  made  of  Elasticity,  Stress  and 
Strain,  and  Fluid  Motion. 

4.  A  clear  conception  of  Work,  Energy,  Power  and  Power  Ef- 
ficiency, is  of  fundamental  importance. 

5.  Graphical  representations,  composition  and  resolution  of  forces, 
are  constantly  of  use. 

6.  Various    modes    of    representing    variations    of    quantities    on 
charts,  serve  as  the  basis  of  recording  air  pressure  results,  and  should 
be  fully  appreciated. 

7.  The  relative  values  and  conversion  factors  of  different  sys- 
tems of  units,  are  most  useful,  and  areas,  volumes,  etc.,  are  frequently 
called  for. 

Recalling  these  elements  is  made  simpler,  if  a  brief  summary  of 
the  features  particularly  applicable  to  this  study  be  given. 


26 
FORMULAE. 

To  attempt  to  present  a  study  of  flight  without  any  formulae 
would  make  it  necessary  to  express  relations  between  quantities  in 
long  paragraphs  of  words,  that  could  more  readily  be  stated  in  simple 
equations. 

There  is  nothing  mysterious  about  an  equation. 

It  is  merely  a  sentence  tersely  expressed. 

Thus,  if  it  was  desired  to  state  the  rule  that  the  quantity  A  mul- 
tiplied by  twice  the  quantity  B  is  equal  to  C,  the  formula  represent- 
ing this  would  be, 

Ax  2B  =  C 

Each  letter  or  symbol  in  a  formula  represents  some  factor  that 
is  substituted  when  its  value  is  known.  If  A  =  16  and  B  =  4,  then 
C  =  128,  since,  the  rule  interpreted,  reads, 

16  x  8  =  128 

Besides  equations,  other  relations  may  be  represented  by  formu- 
lae. Thus,  the  sign  "  °c  ,"  signifying  "varies  as,"  would  permit  the 
statement  that  "wind  pressure  varies  as  the  square  of  the  velocity 
of  the  wind,"  to  be  expressed 

P  oc  V2 

Equations  are  of  two  kinds,  derived  and  empirical.  A  derived 
equation  is  susceptible  of  proof,  by  use  of  mathematical  processes 
based  on  proven  assumptions. 

An  empirical  equation  is  neither  derived  nor  proven.  It  is  merely 
a  statement  of  the  results  of  experiment,  regardless  of  mathematical 
proof. 

In  many  branches  of  engineering,  empirical  formulae  are  con- 
stantly used,  and  in  Aviation,  the  lack  of  a  satisfactory  basic  theory 
of  air  flow  makes  empirical  formulae  based  on  experiment,  most  neces- 
sary. 

Empirical  formulae  are  really  experimental  averages.  As  an 
example:  The  theory  of  long  columns,  has  not  as  yet  permitted  of  the 
mathematical  derivation  of  a  satisfactory  set  of  formulae  for  the  stresses. 
Very  extensive  experiments  have  been  conducted  therefore,  on  the 
loads  necessary  to  deflect  and  break  such  columns.  Grouping  these 
experimental  results  together  it  is  found  that  if  1/d  denotes  the  length 
ratio  of  a  certain  column,  and  p,  the  stress  per  square  inch  of  cross 
section,  the  average  of  the  experiments,  may  be  expressed  as 
p  =  32,000  -  277  1/d 

This  is  strictly  an  empirical  formula.  The  engineer  is  interested 
in  its  practical  application,  not  in  its  derivation,  and  when  a  column 
of  this  type  is  to  be  designed,  for  any  value  or  1/d  he  can  find  the  value 
of  p. 

Formulae  of  empirical  nature  are  fundamental  in  a  study  of  Aviation. 


27 

It  is  often  found  necessary,  particularly  in  an  experimental  field, 
to  introduce  numerical  constants,  to  balance  the  two  sides  of  an  equa- 
tion. It  may  be  known,  for  example,  that  the  horse-power  of  a  pro- 
peller varies  as  the  cube  of  the  revolutions  and  the  fifth  power  of  the 
diameter,  but  we  could  not  express  this  relation  as  an  equation,  capable 
of  solution,  until  a  numerical  factor  is  found  which  gives  a  value  to  the 
h.  p.  (horse-power)  for  any  r.  p.  m.  (revolution  per  minute)  or  diameter, 
that  agrees  with  the  experimental  results. 

Thus  the  relation  could  be  written, 

H.  P.    =  kN3  D5 

but  unless  k  =  1,  the  equation  cannot  be  solved  until  a  value  of  k  is 
found.  Since  the  equation  is  empirical,  it  becomes  necessary,  actu- 
ally to  try  many  propellers,  until  an  average  is  found.  As  a  matter 
of  fact,  k,  in  the  above  formulae,  has  been  determined  by  experiment 
to  be  0.54  when  certain  units  are  used.  The  formula  becomes, 
H.  P.  =  0.54  N3D5, 

and  is  capable  of  simple  arithmetical  solution  by  substituting  values 
for  the  letters.     A  term  like  k  is  called  a  "constant." 

The  majority  of  formulae  for  air  pressures  involve  "constants," 
and  the  great  advance  in  designing  during  the  past  two  years  may  be 
traced  directly  to  the  determinations  by  the  aerodynamic  laboratories, 
of  better  values  of  these  constants,  for  use  in  empirical  formulae. 

SOLVING  TRIANGLES. 

Every  triangle  has  six  parts,  three  sides  and  three  angles,  and  if 
we  know  any  three  (including  a  side)  the  triangle  may  be  solved  — 
that  is  the  other  sides  and  angles  may  be  determined. 

Triangles  may  be  solved  in  two  ways : 

1.  By  trigonometry. 

2.  By  graphical  methods. 

In  aviation  work  only  the  simplest  trigonometry  is  used,  and 
about  the  only  functions  of  angles  used  are  the  sine,  the  cosine  and 
the  tangent.  It  is  well  to  recall,  here,  that  "sine"  and  "cosine"  are 
merely  numerical  ratios,  representing  the  fractions  that  certain  sides 
of  a  triangle  are  to  the  hypothenuse. 

The  accompanying  chart  shows  what  these  functions  are,  and 
also  gives  formulae  for  solving  the  triangles. 

In  later  chapters  it  will  be  found  that  in  the  representation  and 
solution  of  forces,  in  the  determination  of  angles  of  incidence,  glides 
and  climbs,  and  in  stress  determinations,  many  occasions  arise  for 
solution  of  simple  triangles. 

But  in  aeroplane  work  great  accuracy  of  computation  is  not  ne- 
cessary, so  that  a  simpler  way  of  solving  triangles  may,  at  times,  be 
used,  i.  e.,  the  graphical  method.  This  consists  merely  in  a  mechani- 
cal process  of  laying  off  on  a  sheet  of  paper  the  known  angles,  by  a 


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VARIOUS  MATHEMATICAL  SIGNS  -  FORMULAE  FOR  AND  GRAPHICAL 
SOLUTION   OF   TRIANGLES -AND   FUNCTIONS  OF   ARCS 


29 

protractor,  and  the  sides  to  some  convenient  measurement  scale.  By 
closing  the  triangle,  all  that  is  necessary  in  order  to  determine  the 
other  sides  or  angles,  is  to  measure  them  off.  At  first  sight,  this  seems 
to  lack  the  value  of  preciseness,  but  if  a  large  enough  scale  is  used, 
it  is  surprising  how  quickly  and  correctly,  triangles  may  be  solved  in 
this  way. 

In  aeroplane  studies  the  use  of  logarithms  is  rarely  justified  ex- 
cepting possibly  in  propeller  determinations,  where  formulae  involv- 
ing, for  example,  the  fifth  power  of  the  diameter,  D5,  are  used. 

It  may  be  recalled  that  a  logarithm  is  merely  the  exponent,  like 
five  in  the  above,  to  which  it  is  necessary  to  raise  10,  in  order  to  pro- 
duce the  given  number. 

It  will  suffice  to  give  here,  the  method  of  determining  powers  of 
numbers.  For  example,  in  determining  D5,  a  laborious  calculation 
is  avoided  by  looking  up  the  log  of  D,  multiplying  it  by  five,  and  find- 
ing the  number  corresponding  to  the  log  represented  by  this  quotient. 

The  occasion  will  rarely  arise  where  logs  have  to  be  used,  or  even 
trigonometry,  if  graphical  methods  are  pursued. 

MECHANICS. 

Mechanics  is  the  most  logical  of  sciences  —  the  causes  and  effects 
are  so  evident.  It  is  often  defined  as  the  science  that  treats  of  the  ac- 
tion of  forces  upon  bodies.  And  anything  that  concerns  the  action  of 
air  forces  on  aeroplane  wings  and  bodies,  is  of  vital  importance  here. 
It  is  almost  needless  to  recall,  that  as  long  as  the  propeller  is  pulling 
or  pushing,  or  the  aeroplane  gliding,  it  is  storing  up  momentum,  which 
is  defined  as  the  product  of  the  mass  m  by  the  velocity  v  at  any  instant ; 
whereas,  inertia  is  that  property  of  a  body  by  virtue  of  which  it  tends 
to  continue  in  whatever  state  it  happens  to  be,  until  acted  upon  by 
some  other  force. 

Velocity  and  Acceleration. 

Acceleration  of  a  particle  is  the  amount  of  increase  or  decrease 
of  its  velocity  in  a  unit  of  time.  In  other  words,  while  the  velocity 
is  rate  of  motion,  acceleration  is  rate  of  change  by  velocity. 

A  force  is  equal  to  a  mass  multiplied  by  its  acceleration,  because 
it  is  universally  agreed  that  a  force  be  measured  by  its  effect  in  chang- 
ing the  velocity  of  a  particle.  When  we  measure  weights  in  pounds, 
we  actually  measure  the  force  of  the  earth's  attraction,  which  is  equal 
to  the  mass  of  the  body  times  the  acceleration  of  gravity,  g,  which 
increases  the  velocity  of  a  particle  32  feet  per  second  every  second. 

Therefore  w  =  m  x  g.  So  that  when  the  mass  of  a  particle  is 
considered,  it  must  be  recalled  that  it  is  equal  to  what  we  call  the  "weight" 
divided  by  acceleration  of  gravity  or 


A  body  falling  freely  under  the  action  of  the  constant  pull  of  the 
earth,  disregarding  the  retarding  effects  of  air  resistance,  is  an  example 
of  uniformly  accelerated  motion.  It  must  not  be  forgotten  that  in  a 
vacuum  all  bodies,  whether  a  feather  or  a  piece  of  lead,  fall  at  the  same 
speed.  Air  resistance,  alone  affects  rate  of  fall,  in  free  air. 

It  is  useful  to  recall,  that  a  falling  body  attains  a  velocity  v  in 
feet  per  second,  falling  a  distance  h  feet,  represented  by 

v  =  \/2gh 
where  g  =  32  feet  per  second  per  second. 

Rotary   Motion  and  Centrifugal  Force 

In  a  circular  orbit  of  radius  r  a  particle  making  in  revolutions  per 
second,  covers  in  each  revolution  the  circumference,  2  TT  r,  so  that  its 
velocity  in  feet  per  second 

v  =  2  TT  r  n 

The  numerical  value  of  this  velocity  is  solved  by  the  above  equa- 
tion, easily  enough,  but  the  particle  swingng  in  a  circle  is  constantly 
changing  the  direction  of  its  velocity.  This  change  in  v,  involves  an 
acceleration,  and  since  the  particle  has  mass,  it  follows  that  a  force 
is  introduced,  which  is  constantly  making  or  trying  to  make  the  particle 
hold  its  circular  path.  This  is  the  centripetal  force. 

The  force  acting  from  without  and  tending  to  make  a  particle 
take  a  curved  path  is  called  centripetal  force,  and  is  the  opposite  to 
centrifugal  force. 

Since  this  acceleration  towards  the  center  of  the  circle  is  equal 
to  v2/r,  it  follows  that 

V2          W  V2 

Centrifugal  force  F  =  m  x  —    =    — 
r         gr 

where  w  is  the  weight  in  pounds,  v  is  speed  in  feet  per  second,  r  is  the 
radius  of  the  orbit  in  feet  and  F  is  the  force. 

The   Pendulum 

What  applies  to  the  speed  with  which  weights  fall,  applies  also 
to  the  simple  pendulum.  No  matter  what  the  weight  of  the  pendu- 
lum, it  is  the  length  of  arm  1  alone,  that  governs  the  period  of  oscillation. 
This  period, 

P    =    2    7T  V~T7g~ 

Moment   of  Inertia. 

Inertia  has  been  defined,  but  "Moment  of  Inertia"  must  be  con- 
sidered when  we  come  to  rotary  motion. 

Moment  of  inertia  is  the  quantity  obtained  by  multiplying  the 
mass  of  each  particle  of  a  body  by  the  square  of  its  distance  from  the 
axis.  Whether  a  propeller,  a  flywheel,  or  a  wing  spar,  every  object 
has  a  "moment  of  inertia"  I,  about  any  axis.  It  would  be  a  laborious 


31 

computation  to  find  I  for  various  shaped  bodies.  Fortunately  it  has 
been  done  for  us,  and  values  are  given  later  in  a  table.  I  is  expressed 
in  pounds  x  feet  squared  (Ibs.  ft.2). 

Angular   Velocity 

The  "radian"  is  often  used  as  the  measure  of  a  distance  along 
the  circumference  of  a  circle.  There  are  2  TT  or  6.28  radians,  covered 
in  one  revolution  of  a  circle.  So  that  one  revolution  per  second,  r.  p.  s., 
equals  2  TT  radians  per  second. 

If  w  is  called  the  rotational  or  angular  velocity  of  a  particle,  and 
n  the  r.p.m.,  then, 

w  =  2  Trn 

It  has  been  indicated  that  acceleration  of  a  rotating  particle,  due 
to  change  in  direction,  gives  rise  to  centrifugal  force. 

But  the  rotational  velocity  of  a  particle,  may  increase  or  decrease. 
This  is  called  angular  acceleration  and  is  a  rate  of  change  of  angular 
velocity,  called  s. 

Torque. 

In  linear  accelerations,  we  have  Forces,  while  in  rotational  accel- 
eration, forces  are  also  to  be  considered,  but  instead  they  are  called 
Torques. 

Torque,  T,  also  equals  mass  X  acceleration,  but  in  its  case  mass 
is  the  moment  of  inertia  and  acceleration  is  angular. 

1 

.'.  T  =  I  x  s  x  - 
32 

where  T  is  in  pounds  weight  x  feet  and  s  in  radians  per  second  per 
second. 

The   Gyroscope 

Linear  motion  and  rotating  motion  have  been  considered.  The 
axis  upon  which  a  body  is  rotating  can  be  moved  in  a  linear  motion. 

In  addition  the  axis  of  a  rotating  body  may  change  its  direction 
continually.  This  brings  us  to  the  gyroscope. 

An  unbalanced  force  is  of  course  necessary  to  change  the  direc- 
tion of  linear  motion  of  a  particle. 

In  the  same  way  an  unbalanced  force  is  necessary  to  change  the 
direction  of  the  axis  of  a  rotating  body.  When  a  wheel  is  set  rotat- 
ing, the  direction  of  the  axle  tends  to  remain  unaltered,  as  long  as  no 
unbalanced  external  force  acts  upon  it.  But  when  an  unbalanced 
force  is  applied  suddenly  enough  the  axle's  fixed  position  in  space  gives 
rise  to  a  curious  phenomenon,  not  only  resisting  movement  by  this 
force,  but  actually  causing  the  axle  to  move  in  a  direction  at  right 
angles  to  the  applied  force.  It  is  unnecessary  here  to  take  up  the 
relation  of  this  phenomenon  to  the  earth's  rotation  or  the  derived  formu- 
lae, representing  it. 


32 

An  example  of  gyroscopic  force,  however,  may  be  given.  If  a 
bicycle  wheel  is  held  out  in  front  of  one,  by  one  end  of  its  axle,  and 
set  rotating  clockwise  as  viewed  by  the  holder,  when  the  axle  is  pointed 
down  the  tendency  is  for  it  to  swing  around  and  point  to  the  left,  and 
any  effort  to  point  the  axle  upward,  meets  a  pronounced  resistance, 
the  axle  at  the  same  time  turning  sharply  to  the  right. 

The  effect  of  this  phenomenon  on  the  aeroplane's  stability  is  taken 
up  later.  In  steadying  ships  or  monorail  cars,  or  in  stability  devices 
for  aeroplanes,  the  movement  at  right  angles  to  the  direction  of  the 
applied  force  of  a  sensitive  "gyro"  is  made  use  of. 

Elasticity  —  Stress   and   Strain. 

The  phenomena  which  are  associated  with  the  distortion  of  bodies 
due  to  stresses  are  excessively  complicated,  and  one  has  but  to  think 
of  the  many  familiar  properties  of  brittle  substances,  like  glass  or  chalk, 
elastic  ones  like  spring  steel  or  rubber,  and  plastic  ones  like  clay  or 
wax,  to  realize  that  this  is  in  itself  a  formidable  study,  much  too  ex- 
tensive to  be  given  anything  but  a  meagre  consideration  here.  The 
importance  of  the  study  of  Resistance  of  Materials,  to  aviation,  can- 
not be  overestimated,  since  in  the  design  of  the  aeroplane  proper,  this 
is  the  branch  of  engineering  that  solves  the  fundamental  problem  — 
to  build  light  and  yet  strong. 

This  necessary  combination  is  one  that  truly  represents  a  cri- 
terion of  the  excellence  of  an  aeroplane,  as  a  structural  engineering 
unit,  and  although  it  often  does  not,  nevertheless,  the  aeroplane  should 
involve  the  most  refined,  advanced  and  expert,  structural  features 
that  engineering  development  has  made  possible.  It  has  been  a  great 
detriment  to  aviation  that  so  many  of  its  devotees  have  failed  to  realize 
that  the  very  best  material  obtainable,  and  the  most  ingenious  and 
perfect  construction,  is  still  hardly  good  enough  to  bear  the  strains 
properly. 

Of  all  the  great  variety  of  solid  substances,  having  almost  every 
imaginable  degree  of  elasticity,  softness,  hardness  and  brittleness, 
we  are  concerned  in  later  chapters,  only  with  the  behavior  under  stress 
of  those  which  are  used  as  materials  of  construction,  such  as  steel, 
aluminum,  brass,  linen,  spruce,  ash,  glues,  paints  and  rubber. 

Of  the  three  classes  of  substances,  solids,  fluids  and  gases,  let  it 
be  recalled,  that  an  "elastic"  solid,  like  spring-steel,  can  withstand 
a  stress  which  tends  to  change  its  shape  for  an  indefinite  length  of 
time,  whereas  a  "plastic"  solid,  like  wax,  does  not  recover  from  strain 
when  the  stress  ceases  to  act.  One  must  qualify  the  above,  however, 
since  the  best  spring  steel  never  completely  recovers  from  distortion, 
and  even  wax  is  slightly  elastic.  A  fluid  is  a  substance  which  at  rest 
has  no  power  definitely  to  resist  a  stress,  and  when  at  rest  it  is  always 
pressing,  normally,  on  the  sides  of  the  vessel  containing  it.  A  gas  is  a 


matter  with  no  independent  shape,  adjusting  itself  to  take  the  form 
of  the  vessel  in  which  it  is  confined,  and  tending  to  diffuse  and  expand 
indefinitely. 

Substances  are  of  two  kinds  • —  grained  and  ungrained.  Glass 
and  water  are  examples  of  ungrained  substances,  while  wood,  steel, 
and  practically  all  materials  of  construction,  have  a  grained  struc- 
ture. The  grain  in  steel  is  well  marked,  and  though  often  lost  sight 
of,  it  is  most  necessary  in  aeroplane  work,  that  care  be  taken  not  to 
put  too  great  a  stress  across  the  grain  of  a  steel  plate. 

Elasticity  may  properly  be  defined  as  the  resisting  property  of  a 
body  to  motion  of  its  molecules. 

Strain  is  the  distortion  of  a  body  measured  at  a  given  point. 

Stress  is  the  force  by  which  the  molecules  resist  a  strain  at  any 
point.  Stresses  are  developed,  and  strains  caused,  by  the  application 
of  external  forces.  Each  stress  is  accompanied  by  its  own  character- 
istic strain. 

Stresses  are  of  five  kinds  —  Tension,  Compression,  Flexure,  Tor- 
sion and  those  induced  by  Fluid  pressure.  They  are  illustrated  on  an 
accompanying  cut. 

It  is  a  fact  of  fundamental  importance  in  the  theory  of  elasticity, 
that  however  irregularly  a  body  may  be  distorted,  any  small  portion 
of  the  body  suffers  that  simple  kind  of  distortion  which  changes  a  circle 
into  an  ellipse,  the  change  of  shape  consisting  essentially  of  an  increase 
or  decrease  of  linear  dimensions  in  three  mutually  perpendicular  direc- 
tions, sometimes  accompanied  by  a  slight  rotation  of  the  small  parts 
of  a  body. 

The  stress  on  a  body  is  usually  represented  as  pounds  per  square 
inch,  or  the  force  in  pounds  acting  on  a  one-inch  square  part  of  the 
body.  The  total  force  P  on  a  body,  divided  by  area  A,  of  its  cross- 
section  gives  this  unit  stress  which  is  called  "intensity  of  stress."  The 
strain  1  accompanying  this  is  not  represented  in  actual  inches  or  units 
of  total  deflection  d,  but  is  given  as  a  fraction  of  the  span  L  of  the  piece, 
such  that  strain  1  equals  d/L. 

The  basic  law  of  Resistance  of  Materials  is  that  intensity  of  stress 
p  is  proportional  to  strain  1.  And  to  balance  the  proportion  into  an 
equation,  a  constant  is  introduced,  called  E,  giving  the  simple  rule, 

that 

p  =  P/A  =  d/L  x  E  =  1  x  E 

This  constant  E,  is  called  the  "Modulus  of  Elasticity,"  and  is  of 
the  greatest  convenience  in  indicating  what  the  proportion  of  stress 
in  a  given  material  is  to  strain.  Thus,  it  is  readily  seen  that  steel  is 
stronger  than  aluminum,  when  it  is  learned  that  E  for  steel  is  28,000,- 
000  and  for  aluminum  1,700,000. 


l/bnous  hinds  of  Stresses 


faflical  SoMicv  offerers 


_^     /  Iff**  A.  4* 

/,  -Hen,  0'  ><>*> 

YC  **tC*  t<t> 


yiren  faint  -  frrtf  K  trrrr  arm  , 


/  m  JL  dsfarrc?  from   hne  of  octto 

fffanr  tc  fwnt  of  momfriK. 

7fat  rrwmt  of  'P  "»>ok~,  <***  o   .-  J*  on 

ana     "       "C'  '    "          "       "    •    CrOm 

7J?f  immirrt-  rj'  P  a  arrti.  cjcdtwist 


Cembosrftcn  of ' /tares  — 

Xteral  m1o   a.  rauHanT,™ 


orftl  if  on?  fatt 
rf<xi   Jc   ayitrr,    kntfb  e/talff. 

»,!>>  telgr    ift.h 


,,*,  3*,t  9  *,  f-  •><"".  «ol 


ourr    fy    am     * 
•ntl'i'i    at*   Jmwii     ./w/",    ?A,i  are  /**  S  ijotn,  as  *? 
**  ar«    y  y  of  Or*  /.    Ay  mmwv^t  O/.HJ   Ibnt 
h  vckmt  x»/r  on  OUVTr   /. 


KINDS  OF  STRESSES  -  GRAPHICAL  FORCE  DIAGRAMS  -  CHARTS  AND 

GRAPHS 


35 

For  all  materials,  however,  there  is  a  limit  beyond  which  the  ratio 
of  stress  to  strain  or  coefficient  of  elasticity  E  =  p/1,  does  not  hold. 
This  region  is  called  the  "elastic  limit"  of  the  material,  and  while  con- 
siderable stress  can  be  added  beyond  this,  the  material  begins  to  stretch 
out  of  all  proportion  and  rapidly  reaches  the  breaking  away  point, 
which  is  called  the  "ultimate  resistance." 

When  relieved  of  stress,  before  reaching  its  elastic  limit,  a  ma- 
terial will  return  more  or  less  to -its  former  state,  but  when  the  stress 
has  exceeded  the  elastic  limit  the  material  takes  a  permanent  set. 
The  forces  necessary  to  bring  any  material  to  the  elastic  limit,  and 
the  value  of  the  ultimate  resistance,  are  entirely  matters  of  experi- 
ment, from  which  are  derived  empirical  values. 

Fluids  and  Gases. 

In  liquids  the  phenomena  of  surface  tension,  capillary  action, 
cohesion,  etc.,  are  of  but  minor  interest  excepting  in  hydro-aeroplane 
studies.  It  is  important  to  recall  of  liquids,  however,  that  the  pres- 
sure exerted  on  any  part  of  an  enclosed  liquid,  is  transmitted  undi~ 
minished  in  all  directions  (air-pressure  fuel  tanks).  When  a  fluid 
is  in  motion  it  is  being  acted  upon  by  an  unbalanced  force,  giving  it 
velocity  and  by  a  pressure,  or  in  other  words,  it  has  the  energy  of  a 
"velocity  head"  and  a  "pressure  head."  Any  increase  in  one  is  at 
the  expense  of  the  other. 

A  device  very  widely  used  for  the  measurement  of  velocities  of 
both  water  and  air  is  the  Pitot  tube,  which  measures  the  velocity  head 
v  =  v/2gh.  It  consists,  merely  of  a  bent  tube  with  a  nozzle,  point- 
ing into  the  relative  flow  and  measuring  by  means  of  the  length  of  a 
column  of  liquid,  the  head  h,  which  substituted  in  the  above,  gives  the 
velocity  v. 

In  considering  liquids  the  losses  in  head  in  long  pipe  lines  and 
the  effects  of  expansion  and  contraction  and  of  nozzles,  are  of  inter- 
est with  reference  to  the  gasoline  and  radiator  connections. 

Buoyancy  and  Specific  Gravity  should  be  considered. 

A  body  immersed  in  a  liquid  or  a  lighter  gas  immersed  in  air,  is 
acted  upon  by  a  lifting  force  which  equals  the  weight  of  the  liquid  or 
air  displaced.  In  other  words,  the  law  of  Floating  Bodies  is  to  the 
effect  that  a  floating  body  will  displace  a  volume  of  liquid  of  gas  whose 
weight  equals  its  own.  A  body  immersed  in  pure  water  has  a  flota- 
tion of  62.4  Ibs.  per  ft.3 

The  density  of  a  substance  is  its  mass  per  unit  volume,  while  Spe- 
cific Gravity  of  a  substance  is  its  weight  as  compared  with  the  weight 
of  an  equal  "bulk"  of  pure  water.  So  that,  given  the  specific  gravity 
of  a  substance,  it  is  necessary  to  multiply  by  62.4  to  obtain  its  actual 
weight  in  pounds  per  cubic  foot,  since  water  weighs  62.4  Ibs.  per  ft.3 
Specific  gravity  is  sometimes  referred  to  other  substances  —  air  for 


36 

example.  The  specific  gravity  of  gold  is  19.26.  Its  weight  per  cubic 
foot  is  consequently  1,200  Ibs.  A  table  of  weights  and  specific  gravi- 
ties is  given  later. 

Gases  are  highly  compressible,  in  distinction  to  water  and  solids, 
and  are  perfectly  elastic,  though  in  distinction  to  solids  their  elasticity 
is  one  of  volume  and  not  of  form. 

It  must  be  borne  in  mind,  with  reference  to  gases,  that  the  tem- 
perature remaining  the  same,  the  volume  of  a  gas  is  increased  exactly 
in  the  same  proportion  as  the  pressure  is  decreased.  Or,  the  product 
of  volume  X  pressure  equals  a  constant  quantity. 

The  study  of  Aerodynamics  which  constitutes  the  major  part  of 
this  worJc,  takes  up  the  mechanics  of  gases,  making  it  unnecessary  to 
give  them  further  consideration  here. 

WORK,  ENERGY,  POWER. 

Work  is  said  to  be  done  when  a  resistance  is  overcome,  so  that 
movement  takes  place  through  a  certain  distance.  The  air  propeller 
which  pulls  against  a  resistance  of  200  pounds,  causing  the  machine 
to  which  it  is  fixed  to  move  80  feet,  is  doing  work,  inasmuch  as  it  is 
continually  overcoming  this  resistance. 

The  unit  of  work  is  the  foot-pound,  which  is  equivalent  to  the 
work  performed  in  moving  one  pound  of  weight  through  one  foot  of 
space. 

Work  may  be  done  in  several  ways  —  pushing  or  pulling  weights, 
or  working  against  pressures,  such  as  the  work  performed  by  a  piston 
in  driving  a  fluid  of  gas  before  it,  which  is  equal  to  the  intensity  of 
pressure  X  area  of  piston  x  distance  traversed  or  stroke. 

In  the  above  example,  the  propeller  is  doing  16,000  foot  pounds 
work  by  overcoming  a  resistance  of  200  pounds  and  moving  against 
it  80  feet. 

Work,  in  whatever  units  it  is  expressed,  is  always  "resistance 
overcome"  multiplied  by  "distance  traversed." 

Energy  is  distinct  from  work,  in  that  it  represents  capacity  to 
do  work,  but  not  the  actual  work  done.  It  is  expressed  in  the  same 
units  as  work. 

There  are  two  kinds  of  energy  —  Potential  and  Kinetic  —  since  a 
body  when  at  rest  may  have  stored  up  "potential  energy"  due  to  its 
peculiar  position  or  condition,  and  when  in  motion,  a  body  is  capable 
of  performing  work  against  a  retarding  resistance,  due  to  its  "kinetic 
energy." 

A  reservoir  full  of  water,  capable  of  turning  a  water  wheel,  if  re- 
leased, is  an  example  of  potential  energy,  and  another  is  the  stored 
energy  in  storage  batteries  or  gunpowder.  The  weight  of  the  stored 
body  x  the  distance  through  which  it  is  capable  of  acting  is  the  meas- 
ure of  potential  energy. 


37 

Kinetic  energy  or  K.  E.  of  a  body,  is  equal  to  the  work  which  must 
have  been  done  upon  it  to  have  brought  it  to  its  actual  velocity  from 
a  state  of  rest.  While  potential  energy  is  due  to  the  acquirement 
of  "strategical  position,"  kinetic  energy  is  due  to  the  acquirement 
of  "tactical  impetus"  or  velocity. 

Kinetic  Energy  =  wv2/2  g  and  is  derived  from  the  familiar  rela- 
tion v  =  V  2  g  h  since  K.  E.  equals  the  weight  of  the  body  x  height 
from  which  it  would  have  had  to  fall  to  acquire  its  velocity. 

Finally,  it  becomes  obvious  that  Energy  exerted  =  Work  done. 

In  referring  to  the  amount  of  work  done  in  a  unit  of  time,  it  is 
necessary  to  consider  Power,  which  may  be  denned  as  the  rate  of  doing 
work.  Whether  the  propeller  in  the  above  example  traverses  the 
80  feet  of  distance  in  one  second,  or  in  one  hour,  the  actual  work  done 
in  foot  pounds  is  the  same,  since  time  is  not  a  dimension  of  work.  Ob- 
viously, it  would  take  more  "power"  to  overcome  any  resistance  in 
one  second  than  in  one  hour,  and  to  measure  power  it  is  necessary 
not  only  to  consider  the  resistance  and  the  distance  traversed,  but  also 
the  time  it  takes  to  do  it. 

Power,  then,  is  the  number  of  foot  pounds  per  second  or  per  minute 
or  the  number  of  mile-tons  per  year,  if  we  choose  to  use  such  units. 

The  customary  unit  of  power  is  the  Horse-Power. 

One  horse-power  equals  33,000  foot  pounds  per  minute,  or, 
1  h.p.  =  550  foot  pounds  per  second. 

Thus,  when  a  weight  of  5.5  pounds  is  moved  100  feet  per  second, 
one  horse-power  is  exerted. 

An  aeroplane,,  with  a  resistance  in  the  air  of  200  Ibs.,  requires  29 
h.p.  when  travelling  at  80  feet  per  second,  since 
200  x  80  H-  550  =  29  h.p. 

It  is  interesting  to  note  here,  with  reference  to  the  possibility  of 
man- power  flight,  that,  for  a  few  minutes  a  man  can  exert  at  the  limit 
200  ft.  Ibs.  per  second,  and  for  an  hour  about  100  ft.  Ibs.  per  second, 
less  than  l/5th  of  one  horse-power. 

Although  much  energy  is  generated  and  expended,  the  fact  re- 
mains that  the  sum  total  of  all  the  energy  in  the  universe  remains  the 
same.  Mechanical  energy  and  heat  are  converted  one  into  the  other, 
the  heat  of  the  boiler,  taken  from  fuel  coming  from  the  earth,  passes 
into  the  engine  and  into  parts  which  do  work  against  various  kinds 
of  friction,  until  finally  the  sum  total  of  the  mechanical  energy  has 
returned  to  the  earth,  from  whence  it  originally  came,  as  heat. 

The  law  of  the  Conservation  of  Energy  is  the  most  firmly  estab- 
lished of  the  laws  of  mechanics,  and  only  by  the  creation  of  an  addi- 
tional amount  of  energy  in  the  universe,  which  is  impossible  by  any 
known  human  agency,  could  perpetual  motion  be  achieved,  although 
some  magnetic  and  atmospheric  phenomena  may  be  used  very  nearly 
to  approach  it. 


38 
POWER  EFFICIENCY. 

Any  machine,  in  order  to  accomplish  an  amount  of  work  in  a  given 
time,  must  have  work  put  into  it  in  proportion.  Due  to  friction  and 
other  losses,  it  is  always  true  that  the  power  obtained  from  a  machine 
is  not  as  great  as  the  power  put  into  it. 

Now,  call  P,  the  power  delivered  by  a  machine,  and  P'  the  power 
necessary  to  put  into  it,  then  the  ratio  P/P'  will  be  less  than  unity,  ordi- 
narily; it  might  be  equal  to  1,  if  the  machine  were  a  perfect  one  with 
no  losses  but  never  can  it  exceed  one. 

The  ratio  of  the  power  delivered  by  a  machine  and  the  power  it 
used  in  doing  so  is  called  the  Power  Efficiency  of  the  machine. 

We  have  used  above  an  example  of  an  aeroplane,  with  a  flying 
resistance  of  200  Ibs.,  which,  when  it  was  travelling  at  80  feet  per  second, 
required  29  h.p. 

If  the  h.p.  of  the  engine  were  50  h.p.  then  the  efficiency  would  be 
29/50  or  58%. 

It  is  most  important  in  this  study  clearly  to  understand  the  sig- 
nificance of  Power  Efficiency. 

FORCES  REPRESENTED  GRAPHICALLY. 

The  development  of  a  simple  notion  into  an  extensive  science  is 
well  illustrated  in  Graphic  Statics. 

Based  upon  the  elementary  fact  that  a  force  can  be  represented 
by  a  line,  —  long  enough  to  measure  its  magnitude  to  some  convenient 
scale,  and  placed  so  as  to  indicate  the  direction  in  which  the  force  acts 
with  reference  to  some  fixed  point  —  there  has  been  built  up  a  com- 
plete science  of  the  action  of  every  kind  of  force,  and  in  many  cases 
simple  solutions  are  obtained  for  problems  that  would  require  com- 
plicated mathematics. 

For  all  ordinary  engineering  the  numerical  computation  of  the 
characteristics  of  forces  has  almost  entirely  given  way  to  their  determi- 
nation by  machine-like  graphical  methods.  In  later  chapters  the 
particular  application  of  graphical  methods  to  determine  the  stresses 
in  aeroplanes  will  be  taken  up. 

It  will  suffice  here  to  give  a  general  idea  of  how  the  combined 
effect  of  several  forces  can  be  determined,  —  composition  of  forces : 
and  how  a  single  force  can  be  split  up  into  an  equivalent  set  of  forces 
—  resolution  of  forces. 

The  single  force,  that  would  have  the  same  effect  at  a  point  as 
a  set  of  several  forces,  is  called  the  Resultant. 

Referring  to  the  diagrams,  illustrating  the  action  of  forces,  it  is 
indicated  that  two  forces  of  4  and  9  Ibs.  are  acting  at  a  point  o.  It 
is  desired  to  know  what  their  combined  effect  is,  so  that  a  single  force 
could  be  placed  at  o  that  would  resist  their  combined  action. 

The  mechanical  process  of  finding  their  resultant  consists  merely 
in  applying  what  is  often  called  the  "parallelogram  of  forces,"  p.  34. 


Graphically,  the  mechanical  process  is  as  follows:  Lay  off  AB  parallel 
to  the  4-lb.  force,  and  from  A  lay  off  AC  parallel  to  the  9-lb.  force.  Com- 
plete the  parallelogram  to  E,  and  draw  AE.  Then  choose  some  scale, 
such  that  AB  when  actually  measured  on  the  drawing  measures  4  units, 
and  AC  9  units.  With  this  same  scale  measure  AE.  It  scales  about 
10  H  units. 

Therefore,  its  value  is  10^  pounds. 

Its  direction  is  given  by  the  direction  of  AE  so  that  by  drawing 
the  force  through  o,  parallel  to  AE,  and  making  it  10  V^  pounds  long 
to  scale,  we  completely  determine  it  in  direction,  magnitude  and  point 
of  application. 

Finding  the  resultant  of  any  number  of  forces,  whether  co-planar 
or  not,  consists  in  finding  the  resultant  of  two,  then  finding  the  re- 
sultant of  this  resultant  and  one  other,  and  so  on. 


Moments  are  defined  on  the  diagram  as  merely  the  forces  times 
their  perpendicular  lever  arms,  from  the  point  about  which  moments 
are  taken.  If  the  force  is  expressed  in  pounds  and  the  lever  arm  in 
feet,  the  moment  is  in  foot-pounds.  The  unit  is  the  same  as  in  Work, 
but  obviously,  moment  expresses  what  could  be  termed  the  Potential 
Energy  of  the  force. 

Scaling  lever  arms  of  forces,  from  diagrams  to  scale,  is  by  far  the 
easiest  and  quickest  way  to  obtain  them. 

Of  course,  if  a  point  is  in  equilibrium,  all  the  forces  pulling  one 
way  are  balanced  by  forces  pulling  the  opposite  way.  In  the  same 
way  the  sum  of  moments  of  all  the  forces  will  be  zero.  This  is  a  very 
important  conception  to  keep  in  mind. 

The  resolution  of  forces  into  parts  or  complements,  along  given 
directions  or  axes,  is  indicated  in  the  diagram,  and  is,  briefly,  a  reverse 
application  of  finding  the  Resultant. 

The  intricate-looking  but  simply-made  stress  diagrams  of  braced 
frames,  like  bridges,  are  made  of  an  elaboration  of  compositions  and 
resolutions  of  forces. 


In  all  this  graphical  work,  it  is  best  to  appreciate  at  the  outset, 
the  necessity  of  learning  the  mode  of  procedure  of  laying  off  the  lines 
like  learning  to  run  a  machine  and  then  merely  keeping  the  scales  used, 
clear  and  unconfused.  Successfully  to  determine  stresses  it  is  as  un- 
necessary to  know  the  theory  involved,  as  it  is  for  the  average  taxi- 
driver  to  know  the  theory  of  why  certain  mixtures  of  gasoline  and  air 
are  explosive. 


40 

Charts   and   Graphs. 

The  representation  of  the  variation  of  something,  as  a  graph  on 
a  chart,  is  merely  a  convenient  way  of  tabulating  results.  Instead  of 
having  long,  cumbersome  tables,  giving  values,  at  certain  intervals,  it 
is  far  easier  to  represent  them  on  a  chart. 

If  it  is  but  appreciated  that  a  graph  is  a  table  with  values  for  all 
intervals  between  the  limits  indicated,  its  convenience  becomes  very 
evident. 

Diagrams  are  given,  as  an  example,  of  two  types  of  co-ordinates, 
the  Rectangular  and  the  Polar. 

Graphs  are  used  very  extensively  in  studying  Aviation,  and  the 
power  curves  for  Aeroplanes  bid  fair  to  become  as  universal  as  the 
power  curves  for  electric  railway  cars,  etc. 

The  combination  of  several  curves  on  the  same  chart  is  illustrated 
in  the  diagram,  and  consists  merely  in  keeping  the  same  cross  lines, 
but  assigning  to  them  different  scales. 


SEVERAL    VIEWS   OF    STURTEVANT   SEAPLANES    USED    IN    THE    U.    S. 

NAVY 


CHAPTER   IV. 
AIR  RESISTANCES. 


The  Aeroplane,  having  been  described  in  a  general  way,  and  an 
outline  having  been  given  of  the  ordinary  conceptions  of  science  ap- 
plied to  it,  we  can  proceed  with  a  detailed  study  of  its  various  elements. 

In  considering  the  Aeroplane,  three  distinct  features  are  pre- 
sented : 

1.  The  determination  of  the  reactions  of  the  air  on  the  parts  of 
the  moving  machine,  giving  rise  to  resistances,  lifting  forces  and  thrusts. 

2.  The  study  of  the   construction   of   the   machine   to   withstand 
these  forces. 

3.  The  investigation  of  the  stability  and  manner  of  operation  of 
the  aeroplane,  under  the  many  conditions  met  with. 

The  determination  of  air  reaction  requires,  at  the  beginning,  a 
clear  understanding  of  the  nature  of  the  air  and  how  it  may  be  expected 
to  act. 

It  is  well  to  realize  that  lifting  forces  and  thrusts  are  no  more  im- 
portant than  are  the  resistances,  at  the  expense  of  'which  flight  is  ob- 
tained. And  when  it  is  found  that  for  every  ten  pounds  of  air  resistance 
saved  there  can  be  carried  an  additional  load  of  almost  one  hundred 
pounds,  the  significance  of  low  air  resistance  becomes  apparent. 

The  late  Edouard  Nieuport,  builder  of  the  famous  French  mono- 
plane, made  one  of  the  greatest  single  advances  in  aeroplane  construc- 
tion, in  the  past  few  years,  by  his  practical  development  of  aeroplanes 
with  very  low  head  resistance.  And  after  the  introduction  of  his  ideas 
such  rapid  strides  were  made  by  constructors  in  the  improvement 
of  the  aeroplane's  efficiency,  that  load  carrying  capacity  was  almost 
doubled.  Another  lesson  in  the  relative  importance  of  the  resistance 
to  motion  of  an  aeroplane,  is  found  in  the  development  of  high-speed 
racing  machines.  It  had  been  generally  assumed  that  speed  depended 
almost  entirely  on  having  added  power,  but  the  development  of  the 
Deperdussin  monocoques  proved  that  far  better  results  could  be  ob- 
tained by  systematic  refinement  and  reduction  in  the  resistances.  It 
is  needless  to  speculate  on  the  speeds  attainable  in  aeroplanes.  The 
nature  of  air  resistance  and  its  increase  with  speed  as  considered  in 


42 

this  chapter,  will  lead  to  the  realization  that  a  high  speed  record  of  130 
miles  per  hour  is  not  going  to  stand  very  long. 

But  it  is  not  so  much  in  the  attainment  of  higher  speeds  that  we 
are  interested  in  air  resistances,  as  it  is  in  the  reduction  of  the  power 
necessary  to  fly.  While  fuselage  and  nacelle  resistances  are  the  largest, 
attention  must  be  given  to  the  air  resistance  of  wires,  fittings,  struts, 
wheels,  etc.,  the  cumulative  effect  of  which  is  surprisingly  great.  These 
resistances,  however,  are  distinct  from  the  resistance  to  motion  of  a 
wing  that  generates  a  lift. 

The  appreciation  of  the  resistances  of  different  forms  and  shapes 
is  of  great  value  in  the  field  in  determining  their  effect  on  the  efficiency 
of  a  machine,  and  also  on  the  stability,  since  changes  in  resistances 
are  apt  to  affect  the  center  of  air  resistance  of  the  machine,  and  con- 
sequently the  equilibrium  of  the  air  forces. 

Occasions  constantly  arise  in  mounting  bomb-dropping  appar- 
atus, guns  and  other  extra  equipment,  and  in  repair  work,  where  in- 
formation of  this  kind  is  of  value. 


The   Atmosphere. 

The  atmosphere  is  an  ocean,  consisting  of  a  mechanical  mixture 
of  several  gases  with  water  vapor,  and  even  on  the  highest  mountain 
we  are  still  living  at  the  bottom  of  this  ocean.  The  atmospheric  en- 
velope has  a  definite  extent,  and  at  any  point  exerts  a  pressure  which 
is  given  rise  to  by  the  weight  of  the  amount  of  air  above  it.  We  are 
constantly  carrying  around,  therefore,  on  our  shoulders,  on  the  roofs 
or  buildings,  everywhere,  the  weight  of  the  column  of  air  directly  above. 
The  higher  up,  however,  the  less  is  the  weight  of  air,  and,  consequently, 
the  less  the  pressure.  Air  being  compressible  this  increase  in  pressure 
with  decrease  in  altitude  affects  the  weight  of  air  per  cubic  volume. 
We  would  have  quite  an  exact  measure  of  height  in  the  atmosphere, 
in  noting  the  corresponding  pressure,  were  it  not  that  this  pressure  is  also 
affected  by  temperature  and  great  wave  movements  of  the  air  ocean, 
storms  and  winds. 

As  the  temperature  increases  the  density  decreases,  and  the  volume 
of  a  pound  of  air  increases  at  the  same  pressure. 

The  unit  of  atmospheric  pressure  is  the  mean  pressure  of  the  air 
at  sea  level,  at  60°  F.  and  is  called  one  "atmosphere."  It  value  is 
14.7  Ibs.  per  sq.  in.,  and  it  causes  the  mercury  in  the  barometer  to  rise 
30  inches.  Over  one  sq.  ft.,  a  pressure  of  one  atmosphere  is  equiva- 
lent to  a  weight  of  2,116  pounds. 


43 

For  every  1000  feet  increase  in  altitude  the  pressure  decreases 
about  H  Ib.  per  sq.  in.  At  a  height  of  18,500  feet,  atmospheric  pres- 
sure is  one-half  of  that  at  sea  level,  and  at  a  height  of  40  to  50  miles 
the  air  must  be  practically  weightless. 

At  atmospheric  pressure  and  60°  F.,  the  weight  or  density  of  air 
is  .081  Ib.  per  cubic  foot. 

It  is  convenient  to  recall  that  air  is  about  1 /800th  as  heavy  as 
salt  water,  and  14  times  heavier  than  hydrogen. 

Nature  of  Air. 

Since  air  has  weight,  it  follows  that,  as  a  substance,  it  has  inertia 
and  momentum.  The  possibility  of  night  is  due  to  the  tendency  of 
air  to  resist  movement. 

In  addition  to  this,  air  is  very  elastic,  but  at  aeroplane  speeds, 
it  may  be  considered,  theoretically,  as  almost  incompressible,  like  water. 

Air  is  a  "continuous"  medium,  each  particle,  naturally,  tending 
to  hold  together  with  every  other  particle,  and  the  tenuous  manner 
in  which  any  air  disturbance  influences  adjacent  air  filaments  is  beau- 
tifully demonstrated  in  photographs  of  air  flow. 

Disturbances  of  the  air  cause  up  and  down  currents,  complicated 
air  vortices,  aerial  fountains,  waves  and  pulsations,  with  changes  in 
the  velocity  and  direction  of  air  streams;  and  just  as  water  boils  so 
will  air  boil,  when  heated.  The  action  of  the  sun  in  boiling  the  air 
over  a  dry,  open  space,  can  be  distinctly  felt  when  flying. 

In  the  consideration  of  air  resistances,  however,  it  is  assumed 
that  the  air  is  uniform  in  flow,  and  at  60°  F.,  and  atmospheric  pressure. 

There  is  another  very  important  conception,  with  regard  to  air 
resistance  determinations.  Disregarding  the  effects  of  inertia  and 
acceleration  of  an  object,  the  air  pressures  are  the  same  in  action, 
whether  the  object  is  moved  against  the  wind,  or  the  wind  against 
the  object. 

Motion  through  the  air  gives  rise  to  two  distinct  kinds  of  resis- 
tance : 

1.  Pressure,  generated  by  the  impact  of  the  air  on  an  object,  and 

2.  Friction,  generated  by  the  flow  of  the  air  filaments  past  the 
surface  of  the  object. 

Characteristics   of  Air  Flow. 

Having  defined  air,  the  manner  in  which  it  flows  may  be  con- 
sidered. Air  either  flows  smoothly  past  an  object  in  stream  lines  — 
continuous  filaments  — •  or  it  breaks  up  into  swirls  and  eddies,  due  to 
too  abrupt  a  change  in  flow.  The  accompanying  photographs  of  air 
flow  illustrate  this. 


44 


PHOTOGRAPHS    OF    THE    EIFFEL    LABORATORY    IN    PARIS,    SHOWING 
THE   TESTING   ROOM   AND   THE   TWO   WIND   TUNNELS 


ASPLCJ  RKTIQ  =  7 


THE   FLOW   OF   AIR 

UPPER  LEFT,  A  FLAT  SURFACE  -UPPER  RIGHT,  A  SPHERE  -LOWER 
LEFT   AND    RIGHT,   STRUTS   OF    DIFFERENT   FINENESS    RATIO 


45 

It  is  apparent  that  a  spindle  or  fusiform  shape,  gently  dividing 
the  air  at  the  front,  and  gradually  permitting  the  filaments  to  close 
together  at  the  rear,  will  give  a  smooth  flow,  which  amounts  to  the 
same  thing  as  a  very  low  resistance.  It  is  also  evident  that  a  flat  sur- 
face creates  very  great  disturbance,  and  consequently  high  resistance. 

The  curve  of  the  stream  lines,  necessary  to  prevent  disrupting 
them,  may  be  computed  for  any  speed,  by  applying  fluid  dynamics. 
But  it  must  be  kept  in  mind  that  a  form  of  this  kind  gives  its  low  re- 
sistance, only  at  one  particular  speed,  since  the  path  of  flow  is  affected 
by  the  speed.  It  is  unnecessary  here  to  take  up  the  determinations 
of  these  forms.  If  the  stream  lines  flow  smoothly  past  an  object,  and 
close  up  again  without  eddies,  it  follows  that  the  only  resistance  ex- 
perienced is  frictional. 

There  are  many  ways  of  determining  the  manner  in  which  the  air 
flows  past  an  object,  such  as  noting  the  directions  in  which  light  silk 
threads  are  blown,  or  introducing  smoke  or  particles  into  the  air  and 
photographing  it.  Ammonium  Chloride  is  a  very  convenient  smoke. 


Importance   of   Visualizing  the   Air. 

It  is  of  great  value  in  aeroplane  work,  to  become  accustomed  to 
visualize  the  streamline  flow  of  air,  and  ability  to  "see  the  air"  often 
solves  many  problems  of  stability  and  reduction  in  resistance,  with- 
out any  recourse  to  mathematics  or  measurements.  Besides  this, 
there  is  offered  in  the  study  of  air  flow  by  photography,  a  field  of  in- 
vestigation of  great  promise  and  absorbing  interest. 

It  is  a  common  experience  that  in  a  wind,  at  the  front  of  a  flat 
surface,  there  is  a  dead  region  of  air,  where  no  wind  is  felt.  Photo- 
graphs show  this  air  cushion  clearly,  and  in  Chapter  VI  this  simple 
conception  is  found  to  hold  a  valuable  theory. 

In  stability  discussions,  effect  of  following  planes,  interference, 
and  propeller  stream  action,  priceless  secrets  would  be  revealed  if  the 
air  could  be  followed  in  its  every  movement. 


Determination   of   Air  Resistance. 

The  nature  of  the  action  of  air  on  objects  has  been  considered, 
but  we  must  know  in  addition  with  what  force  in  pounds  P,  the  air 
pushes  on  an  object  when  it  passes  it  at  velocity  V. 

Applications  of  Theory  to  determine  the  magnitude  of  air  pres- 
sures, are  given  consideration  in  Chapter  VI,  but  merely  for  reference, 
since  the  best  measures  of  air  resistance  have  been  obtained  by  actual 
experiment. 


46 

Methods  of  measuring  the  resistance  of  the  air  that  have  been 
widely  used,  are  the  following: 

1.  Dropping   surfaces   from   a   height    and    measuring    time 
of  drop  and  pressure,  used  by  Newton,  and  Eiffel  in  his  earliest 
experiments. 

2.  The  whirling  arm,   used  by   Langley,   and  consisting  of 
whirling  the  surface  at  the  end  of  a  large  arm  around  a  circle  of 
large  diameter  and  recording  the  resistance  automatically. 

3.  The  moving  carriage,   an  automobile,   trolley   or  car,   as 
used  in  the  experiments  of  the  Due  de  Guiche,  Canovetti,  and  the 
Zossen  Electric  Railway  tests. 

4.  By   blowing  or  drawing  air  through   a  tunnel  in  which 
the  object  or  a  model  of  the  object  is  placed.     This  method  is 
the  most  modern  and  convenient,  and  permits  of  a  uniformity  of 
the  air  current,  which  cannot  be  obtained  as  easily  in  the  open. 

In  wind  tunnels,  the  best  practice  is  to  draw  the  air  in,  through 
screens  and  channels,  that  straighten  it  out,  past  the  experimental 
chamber,  and  thence  to  the  fan.  Practically  all  the  great  Aerodynam- 
ical Laboratories  use  the  wind  tunnel  method  of  experiment.  The 
prominent  ones  are,  the  Eiffel  laboratory  in  Paris,  the  National  Physi- 
cal Laboratory  in  England,  and  the  tunnel  at  the  University  of  Goet- 
tingen.  The  speed  of  the  wind  in  the  Eiffel  laboratory  can  be  brought 
up  to  almost  90  miles  per  hour  (40  metres  per  second),  and  its  size 
permits  of  testing  many  objects  such  as  struts,  to  full  size,  and  complete 
models  of  aeroplanes  to  one-tenth  full  size.  Such  a  magnitude  per- 
mits of  exceedingly  valuable  determinations,  and  the  work  of  the 
laboratories  is  daily  being  applied  with  entire  success  to  full-sized  aero- 
planes. 

It  must  be  borne  in  mind,  however,  that  the  air  in  a  tunnel  is  con- 
fined and  that  all  tunnel  results  are  not  perfectly  adaptable  to  machines, 
unless  suitable  corrections  are  applied. 

Measurements  made  in  the  laboratories  consist  of  determining 
not  only  the  magnitude,  direction  and  position  of  the  wind  forces,  but 
also  in  determining  the  distribution  of  air  pressure  over  an  object  by 
measuring  the  pressures  at  different  points. 

Air  Resistance   varies   as  V2. 

It  has  been  found  by  very  careful  and  extensive  experimenting 
that  the  resistance  of  an  object  in  an  air  stream  is  proportional  to  the 
square  of  the  velocity  of  the  air. 

In  other  words,  if  the  velocity  is  doubled,  it  follows  that  the  re- 
sistance will  be  increased  four  times,  or  if  velocity  is  five  times  as  great, 
the  force  on  the  same  object  would  be  twenty-five  times  as  great. 


47 

There  are  variations  from  this,  however,  due  primarily  to  the 
fact  that  friction  resistance  alone,  as  distinct  from  impact  resistance, 
varies  as  V1'8  increasing  in  less  proportion  than  V2.  On  very  large 
surfaces,  and  particularly  on  dirigible  balloons,  of  streamline  shape, 
the  frictional  part  of  the  resistance  is  by  far  the  greatest,  and  conse- 
quently makes  the  total  resistance  increase  in  a  proportion  less  than 
V2. 

For  our  purposes,  however,  the  total  resistance,  of  objects,  in- 
cluding the  pressures  and  frictions,  are  considered  as  varying  with  V2. 

Air  Resistance   varies   as   S. 

The  size  of  the  surface  area,  on  which  the  air  acts,  S,  gives  a  mag- 
nitude of  air  resistance  that  is  in  direct  proportion  to  the  size.  If  the 
area  of  the  object  is  doubled,  the  air  resistance  is  doubled,  at  the  same 
air  speed. 

This  experimental  fact  is  also  subject  to  modification,  since,  as 
the  size  of  surface  increases,  the  pressures  are  somewhat  greater  in 
proportion.  But  we  can  disregard  this  also  without  serious  error. 

Formula  for  Air  Resistance. 

It  follows,  therefore,  from  the  above,  that  if  we  call  P  the  force 
generated  by  the  air  movement  at  velocity  V  against  an  object  of  area 
S  in  cross-section,  then  P  varies  as  SV2. 

This  at  once  leads  to  an  empirical  formula,  for  the  air  resistance, 
if  we  introduce  K  to  represent  a  numerical  constant,  which  must  be 
determined  for  any  particular  shape  by  experiment. 

It  may  be  stated  then,  that 

P  =  K  S  V2 
This  is  the  fundamental  formula  of  Aerodynamics. 

The  units  used  will  be  S  in  square  feet,  V  in  miles  per  hour  and 
P  in  pounds. 

Although  P  also  depends  on  the  density  of  the  air,  sea  level  and 
60°  F.,  conditions  are  considered  here  and  included  in  the  value  of  K. 

In  this  chapter  we  are  interested  in  the  air  resistance  of  various 
objects  and  parts  made  use  of  in  flying  machines  —  and  in  adding 
to  the  air  resistance  of  these  parts  the  air  force  on  the  wings,  that  must 
be  overcome  to  obtain  the  lift,  we  obtain  the  value  of  the  total  resist- 
ance to  motion  that  is  overcome  by  the  propeller  thrust. 

In  view  of  the  above  formula,  it  becomes  necessary,  merely,  to 
review  and  average  up  the  laboratory  results,  so  as  to  obtain  values 
of  K  for  the  various  different  objects. 

The  most  accurate  determinations  of  the  latest  experiments  are 
made  use  of  for  this  purpose  and  it  is  again  emphasized  that  the  values 
of  K  given,  include  both  the  impact  and  frictional  resistances. 


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THE   RESISTANCE  OF   VARIOUS  SURFACES  AND   BODIES 


Definitions. 

In  Aerodynamical  studies  it  has  become  customary  in  defining 
objects  to  use  unfamiliar  terms. 

Aspect  Ratio  —  is  a  term  used  to  define  the  shape  of  a  surface, 
and  is  the  long  span  of  the  surface  across  the  wind  divided  by  the  width. 

Fineness  Ratio  —  is  a  term  used  to  define  the  general  shape  of 
bodies,  and  is  obtained  by  dividing  the  fore  and  aft  length  of  the  body 
by  the  greatest  width  across  the  wind. 

Master  Diameter  —  is  the  greatest  width  of  a  body  across  the 
wind. 

Fairing  —  is  used  to  denote  the  additional  "tail"  or  filler  used  to 
make  a  poorly  shaped  body  more  streamline  in  form,  thereby  reducing 
its  resistance. 

Diametral  plane  —  is  the  plane,  passed  through  a  body,  facing 
the  wind  perpendicularly,  and  cutting  through  at  the  master-diameter. 

Normal  plane  —  is  another  expression  for  diametral  plane,  and 
merely  refers  to  the  maximum  cross-sectional  projection  of  the  body. 
It  also  refers  to  a  flat  surface  held  normal  (perpendicular)  to  the  air 
current. 

Equivalent  Normal  Plane  — is  the  size  of  normal  flat  surface,  that 
would  give  the  same  resistance  as  does  the  body  referred  to. 

It  has  been  customary  to  refer  to  the  air  resistance  of  all  bodies, 
as  a  percentage  of  the  resistance  of  a  flat  square  normal  surface  under 
the  same  conditions. 

In  this  study,  no  such  conception  will  be  used,  since  values  of  K 
for  each  particular  body  are  studied,  and  the  flat  square  normal  plane 
or  surface  is  merely  considered  as  one  of  several  kinds  of  air-resist- 
ing bodies.* 

Flat  Surfaces, 

Normal  to   the   Air   Stream. 
Square   Planes  — 

In  square  planes,  normal  to  the  air,  the  value  of  K  is  .003  for  sur- 
faces up  to  two  or  three  feet  square,  and  .0033  for  very  large  surfaces 
like  the  sides  of  buildings. 

It  may  be  stated,  therefore,  for  aeroplane  usage,  that  P,  the  air 
resistance  in  Ibs.,  of  a  square  surface,  S  sq.  ft.,  in  area,  at  a  velocity 
V  miles  per  hour,  is 

P  =  .003  S  V2 

Thus,  for  a  surface  2  feet  square,  at  70  miles  an  hour : 
P  =  .003  x  4  x  4900 
P  =  58.8  pounds 

*  Attention  is  invited  to  the  author's  work  "Monoplanes  and  Biplanes,"  Chapt.  II, 
where  a  discussion  of  experimental  results  and  many  values  of  K  are  given. 


50 

In  curve  No.  1  p.  48,  the  graph  gives  values  of  P  in  Ibs.  per  sq. 
ft.  for  speeds  up  to  120  m.  p.  h.  In  the  above  example,  at  70  m.  p.  h. 
the  graph  gives  P  =  14.7  Ibs.  per  sq.  ft.,  or  14.7  x  4  =  58.8  Ibs.,  since 
S  =  4  sq.  ft. 

Rectangles  — 

The  aspect  ratio  of  a  square  is  one.  Rectangles  have  aspect  ratios 
above  one,  when  presented  normally  to  the  air. 

Up  to  an  aspect  of  5  or  6,  K  remains  about  .003. 

An  increase  in  the  value  of  K  is  found  for  rectangles  as  the  aspect 
ratio  increases. 

When  the  aspect  ratio  of  the  rectangles  increases  to  15,  K  becomes 
.0035  and  on  further  increasing  the  aspect  ratio  to  30,  K  =  .0038.  This 
is  shown  on  the  graph,  p.  48. 

A  flat  rectangle,  perpendicular  to  the  air  current,  with  its  dimen- 
sion across  the  current,  thirty  times  as  large  as  its  width,  might  be 
met  with  in  rods,  temporary  struts,  etc.,  and  it  is  interesting  to  note 
how  high  the  resistance  would  be. 

Discs  — 

The  shape  of  flat  surfaces  also  affects  their  air  resistance.     Pass- 
ing from  a  square  plane  to  a  round  disc,  reduces  K  to  .0028,  so  that 
the  air  resistance  of  a  disc  2  feet  in  diameter,  at  60  miles  per  hour,  is 
P  =  K  S  V2  =  .0028  x  .7854  x  4  x  3600 
P  =  32  pounds 

In  general  rounded  edges  may  be  expected  to  reduce  K,  for  flat 
surfaces. 

Parallel    Normal   Surfaces  — 

Discs  or  flat  rectangles,  placed  one  in  front  of  the  other,  interfere 
with  each  other  and  exhibit  a  most  important  phenomenon.  When 
the  discs  are  separated  by  more  than  two  diameters,  both  receive  pres- 
sure; there  is  a  pressure  on  the  front  disc  somewhat  greater  than  on 
a  single  disc,  K  =  .0031,  and  a  very  slight  pressure  on  the  rear  disc. 
But  with  spacing  less  than  this,  the  rear  disc  ceases  to  have  any  pres- 
sure, and  instead  undergoes  a  suction  effect,  which  action  actually 
pushes  it  toward  the  front  disc.  The  forward  push  of  the  rear  disc 
naturally  reduces  the  total  resistance  of  the  two  discs  to  a  smaller  value 
of  K,  making  it  much  less  than  a  single  disc,  when  the  rear  one  is  1 J^ 
diameters  back  of  the  front  one.  This  phenomenon  is  given  rise  to 
by  the  nature  of  the  air  flow,  which  is  illustrated  in  the  diagram.  A 
familiar  application  of  this  is  where  the  racing  bicycle  rider  follows 
in  the  wake  of  a  motorcycle  pace-maker. 

Various   Shaped   Bodies. 
Cylinders  — 

Passing  from  the  disc  to  the  cylinder,  with  the  circular  base  facing 
the  wind,  the  resistance  is  found  to  be  less  as  the  length  of  cylinder 


51 

is  increased,  until  the  length  becomes  greater  than  5  diameters,  when 
the  resistance  is  found  to  increase  again.  Some  values  of  K  are  given 
on  the  chart.  K  for  a  cylinder  7  diameters  long  is  .002. 

When  this  cylinder  is  capped  by  hemispherical  ends,  the  value  of 
K  falls  to  .0006,  an  interesting  result. 

When  the  cylinders  are  stood  upon  their  bases  various  values 
of  K  are  given.  It  is  most  important  to  point  out  that  for  the  two 
cases  corresponding  with  high  aspect  ratio  —  the  cylinder  with  height 
very  low  in  comparison  to  the  diameter,  and  the  long  cylinder  with 
diameter  very  small  in  proportion  to  height  —  the  values  of  K  are  high. 

Wires  and  cables  are  merely  very  long  cylinders.  Extensive  ex- 
periments have  been  conducted  on  them,  and  values  of  K  found.  For 
smooth  wires  K  =  .0026,  whereas  cables  are  found  to  have  considerably 
higher  resistance  with  K  =  .003. 

Thus,  a  machine  having  200  feet  of  1/8  inch  cable,  giving  a  pro- 
jected area  of  200/96  =  2.08  sq.  ft.,  will  have  an  air  resistance  due  to 
the  cables  at  80  miles  an  hour  of 

P  =  .003  x  2.08  x  6400 
=  40  Ibs. 

This  high  value  immediately  suggests  the  advisability  of  reduc- 
tion of  cable  resistances.  In  double  cables,  it  would  prove  beneficial 
to  tape  them  together,  so  as  to  streamline  each  other.  A  graph  is 
given  showing  the  reduction  in  resistance  due  to  inclining  the  wires 
i.  e.,  staggered  planes. 

Experiments  indicate  that  the  vibration  of  wires  does  not  increase 
their  resistance. 

Spheres  — 

The  resistance  of  the  air  on  spheres  presents  a  study  of  interest. 
The  sphere  is  the  simplest  geometrical  form,  and,  as  a  basic  one,  it 
should  long  ago  have  served  as  the  unit  form  for  air  resistance.  Lack 
of  agreement  in  the  experimental  results  of  different  laboratories  was 
only  cleared  up  when  Eiffel  discovered  that  an  increase  of  speed  of  the 
air  above  20  miles  per  hour  caused  a  change  of  flow,  due  to  the  flatten- 
ing out  of  vortices  back  of  the  sphere,  which  reduced  the  resistance 
considerably.  And  that  above  this  speed,  the  nature  of  the  air  re- 
sistance remained  constant.  K  =  .00044,  for  a  sphere,  at  speeds  above 
20  miles  per  hour,  whereas  at  very  low  speeds  K  becomes  .001.  In 
having  a  smoother  flow  at  the  higher  speeds,  less  Ibs.  of  air  are  put 
in  motion,  which  means  that  the  resistance  is  less.  This  action  of  air, 
in  tending  to  smoother  flow  with  speed  increase,  is  important  to  bear  in 
mind. 

For  a  hemisphere,  convex  side  to  the  wind,  K  =  .00083,  and  when 
turned  so  as  to  present  the  concave  side  to  the  wind,  K  increases  to 
.0038. 


res.  of  ont  disc  by  tfatH 
indicate   direction    of 


The  bodies  are  placed  in 
the  order  of  their  least  resist- 
ances. 

For  the  top  one  K  =  .00012 
For  the  lower  one  K  =  .0002 


Bodies 


some    Masler  Diameter  oncf  area  far 
h  different    fids    find     tenths  y  qs  iesiect, 
by  ftffeJ,     X  wry/iy  OS  follows 

\H>1h  speed 
0        16         Zo          36         do       SO        (,D        76       60       96  m  fj.f) 


.000* 


TOP  LEFT  -  INTERFERENCE  OF  FOLLOWING  DISCS  -  TOP  RIGHT,  THE 

BODIES  TESTED  AT  GOETTINGEN  -  BELOW,  BODIES  TESTED 

BY  EIFFEL. 


53 

Streamline  Shapes — 

In  this  class  may  be  included  bodies  of  fusi-form  or  streamline 
form,  shaped  for  least  resistance.  Their  application  to  the  design  of 
tanks,  fuselages,  nacelles,  hoods,  etc.,  is  of  fundamental  importance. 

In  a  most  interesting  set  of  experiments,  conducted  by  M.  Eiffel, 
on  streamline  shapes,  illustrated  in  the  diagrams  and  chart  on  p.  52, 
the  bodies  consist  of  a  nose,  a  cylindrical  central  portion,  and  a  tail. 
The  results  of  the  experiments  show  that : 

1.  The  blunter  the  nose,  the  greater  the  resistance. 

2.  The   shorter  the   central  cylindrical  portion  is,   for  the  same 
nose  and  tail,  the  lower  the  resistance. 

3.  The  effect  of  shortening  up  the  tail  is  not  very  great,  although 
slightly  increasing  the  resistance. 

In  each  case,  however,  measurements  made  at  speeds  up  to  90 
miles  an  hour  showed  that  the  resistance  does  not  vary  as  V2,  the  value 
of  K  becoming  constantly  less  with  speed  increase.  This  is  a  very  sig- 
nificant determination,  and  may  be  explained  on  the  ground  that, 
in  bodies  of  this  kind,  the  major  part  of  the  resistance  at  high  speeds 
is  frictional  and  therefore  increases  at  much  less  than  V2.  In  addi- 
tion the  effect  of  velocity  increase  is  to  flatten  out  the  flow  and  suppress 
eddies. 

The  values  of  K  for  these  bodies  are  given. 

The  Goettingen  Laboratory  conducted  extensive  experiments  on 
the  best  shapes  for  dirigible  balloons  which  it  is  important  to  consider. 
The  models  tested  measured  3.75  feet  long  and  .62  feet  in  diameter, 
giving  a  fineness  ratio  of  6.  The  shapes  in  their  order  of  least  resist- 
ance and  values  of  K  for  25  m.  p.  h.  are  given.  At  higher  speeds,  still 
lower  Ks  would  be  expected. 

The  form  No.  1,  having  the  least  resistance,  is,  perhaps,  the  best 
form  that  has  ever  been  tested  in  a  laboratory,  and  at  high  speeds 
would  give  a  resistance  about  l/25th  of  the  normal  pressure  on  its 
diametral  plane.  It  is  the  form  used  in  the  Parseval  non-rigid  diri- 
gibles. 

It  is  interesting  to  note  in  studying  low  resistance  bodies,  how 
closely  they  resemble  the  shapes  of  fishes,  and  of  birds,  measurements 
of  a  fast  swimming  fish  showing  an  almost  exact  resemblance  to  this 
Parseval  shape. 

As  a  general  rule,  the  best  streamline  body  is  the  one  having  a  fine- 
ness ratio  of  6  and  with  the  master  diameter  about  40%  back  of  the 
nose,  both  nose  and  tail  being  fairly  well  pointed. 
Struts— 

The  application  of  fineness  ratios,  and  shapes  of  least  resistance, 
to  improvement  in  the  form  of  struts,  has  in  many  instances  tremen- 
dously improved  the  performance  of  aeroplanes. 


*l  NPL 


THE   RESISTANCE  OF   SEVERAL   STRUTS  OF    DIFFERENT  SHAPE 


55 

In  addition  to  the  form  for  least  resistance,  however,  the  weight 
of  the  struts  and  their  strength  are  factors  that  must  be  considered 
in  choosing  the  best  shapes.  We  will  confine  ourselves  here,  how- 
ever, to  a  study  of  the  resistance  of  various  shapes. 

A  group  of  strut  sections  are  given  and  K  for  each  one.  It  is 
to  be  noted  that  the  effect  of  yawing  is  greatly  to  increase  these  resist- 
ances by  presenting  the  strut  sidewise  to  the  air,  and  it  will  be  neces- 
sary later  to  consider  the  amount  of  this  increase. 

Inclining  the  strut  to  the  vertical,  as  in  staggered  planes,  has  the 
effect  of  increasing  the  length  of  section  in  the  air  stream,  and,  con- 
sequently, the  resistance  does  not  decrease  for  streamline  shapes,  while 
for  blunter  shapes,  inclination  reduces  the  resistance  considerably. 

In  struts,  as  in  bodies,  an  increase  of  velocity  is  accomplished 
by  a  reduction  in  the  value  of  K,  that  is  more  noticeable  the  greater 
the  fineness  ratio,  i.  e.,  the  longer  the  section  of  the  strut.  This  is 
again  due,  probably,  to  the  preponderance  of  friction  in  the  total  re- 
sistance. 

The  results  obtained  in  studies  of  strut  resistance  indicate  the 
importance  of  having  struts  well  made  and  of  a  uniform  section.  Just 
as  in  bodies,  abrupt  changes  in  contour  must  be  avoided  and  atten- 
tion paid  to  a  smooth  curve  on  either  side  of  the  central  portion. 

It  is  found,  in  general,  that  a  fineness  ratio  of  5  to  1  is  best  for 
use,  where  a  fin  effect  is  desired,  and  where  not,  — -  the  best  fineness 
ratio  is  3  to  1. 

Wheels  - 

The  air  resistance  of  chassis  wheels  is  a  considerable  item  in  flight. 

Experiments  have  been  conducted  on  various-sized  wheels,   and  the 

results  are  as  follows : 

28^"  diameter  by  2^"  tire,  K  =  .0025 
24    "         "          "     3    "     "     K  =  .00265 
21    "         "          "     3   "     "     K  =  .0018 
18    "         "          "     2   "     "     K  =  .0021 

When  the  wheels  are  covered  in,  it  is  found  in  almost  every  case 
that  the  resistance  is  halved,  so  that  for  the  24"  x  3"  wheel,  when 
covered  in,  K  =  .00133.  An  average  K  for  wheels  would  be  .002. 

As  an  example,  it  is  desired  to  determine  the  resistance  of  two 
26"  x  4"  wheels  at  80  m.  p.  h. 

The  projected  surface  =  1.4  sq.  ft. 

/.   P  =  .002  x  1.4  x  6400 
=  18  Ibs. 

If  the  wheels  were  covered  in  at  this  high  speed,  about  9  Ibs.  would 
be  saved  in  resistance;  this  would  permit  of  carrying  about  60  Ibs. 
more  load  on  an  efficient  machine,  or  would  add  10  gallons  more  fuel. 


56 

Fuselages  and  Empennages  — 

The  resistances  of  the  bodies  of  aeroplanes,  and  of  the  tail  pieces, 
constitute  the  major  part  of  the  resistance,  and  their  importance  and 
variations,  with  angles  of  yawing  and  pitching,  make  it  necessary  to 
give  them  separate  consideration  in  a  later  chapter. 

It  may,  however,  be  pointed  out  that  the  data  on  streamline  bodies 
given,  is  readily  applied  to  fuselages.  The  laboratories,  however, 
have  studied  complete  aeroplane  models  and  fuselages,  and  have  ob- 
tained valuable  results. 

Summary. 

The  data  given  in  this  chapter  enables  the  air  resistance  of  vari- 
ous shaped  bodies  to  be  computed  for  any  speed  V  and  any  size  sur- 
face S,  where  S  is  the  maximum  cross-sectional  projection  of  the  body, 
perpendicular  to  the  air  stream.  It  is  merely  necessary  to  supply 
the  numerical  values  of  K,  S  (in  sq.  ft.),  and  V  (in  m.  p.  h.),  in  the 
formula 

P  =  K  S  V2 

It  is  well,  again  to  recall  that  the  propeller  of  an  aeroplane  must 
give  a  pull  or  push  great  enough  to  overcome : 

I.  The  resistance  to  motion  of  the  struts,  wires,  body,  wheels, 
fittings,  skids,  gas  tanks  and  other  attachments. 

II.  The  dynamic  resistance  of  the  wings  and  rudders,  called  the 
Drift  and  generated  by  the  same  pressure  that  gives  the  Lift. 

In  this  chapter  the  first  has  been  considered.  And  a  study  of  the 
second  may  now  be  taken  up. 


A  TRACTOR  AEROPLANE   CLIMBING 


CHAPTER  V. 
INCLINED    SURFACES. 

In  order  to  understand  the  mechanics  of  flying  it  is  necessary 
to  have  a  sound  conception  of  the  nature  of  air  pressure  on  inclined 
surfaces.  On  a  plane  presented  to  the  relative  air  current,  at  an  angle 
less  than  90°,  the  generated  air  pressure  instead  of  acting  straight  back 
is  inclined  above  or  below  the  line  of  flow  of  the  air. 

Before  discussing  this,  however,  a  few  unfamiliar  terms  need  to 
be  defined. 

Span  is  the  dimension  of  a  surface  across  the  air  stream. 

Leading  edge,  is  the  first  edge  of  the  surface  upon  which  the  air 
impinges,  whereas,  trailing  edge,  is  the  rear  edge  of  the  surface. 

Chord,  is  the  dimension  between  the  leading  edge  and  the  trailing 
edge  of  a  surface.  It  is  the  depth  of  surface  along  the  air  stream. 

Surfaces  are  of  two  kinds — flat  in  section  and  curved  in  section. 

Camber,  is  the  rise  of  the  curved  contour  of  an  arched  surface, 
above  the  chord  line. 

It  follows  from  the  above  that  for  any  inclined  surface, 

Span 


Aspect  Ratio 


Chord 


The  explanatory  diagrams  on  p.  60,  are  referred  to,  and  it  is  seen 
that  any  inclined  surface,  is  one  in  which  the  chord  is  inclined  to  the 
line  of  flow  of  the  air. 

This  angle  of  inclination  of  the  chord  to  the  air  stream  is  termed 
angle  of  incidence. 

If  the  leading  edge  of  a  surface  is  presented  to  the  air,  above  the 
trailing  edge,  the  angle  of  incidence  is  said  to  be  positive.  And  when 
the  surface  is  inclined  negatively  to  the  air  flow,  it  is  meant  that  the 
air  impinges  on  the  top  face  of  the  surface,  since  the  leading  edge  is 
below  the  trailing  edge. 


58 

Lift  and   Drift. 

The  air  acting  on  a  surface  presented  to  it  with  a  positive  angle 
of  incidence  generates  a  pressure,  the  line  of  action  of  which  is  pointed 
upwards  and  at  the  same  time  somewhat  backwards.  As  the  incidence 
of  the  surface  is  varied,  of  course,  the  inclination  of  this  force  above 
the  horizontal  is  varied.  But  the  important  conception  to  grasp  is, 
that  the  effect  of  inclining  the  surface  below  90°,  is  to  cause  the  total 
air  pressure  to  assume  an  inclined  position,  with  respect  to  the  axis 
of  flow  of  the  air. 

If  the  inclination  is  such  that  the  total  pressure  points  upward 
and  backward,  a  study  of  the  resolution  of  forces  teaches  that  the  verti- 
cal portion,  or  component,  is  equivalent  to  a  force  acting  vertically 
upwards,  capable  of  lifting  weights,  whereas  the  horizontal  compo- 
nent of  the  same  total  air  pressure  is  a  resistance  to  motion. 

It  follows  that  in  order  to  obtain  this  lifting  component  the  hori- 
zontal one  must  be  overcome,  the  two  together  corresponding  to  the 
resultant  total  pressure  on  the  inclined  surface. 

Lift  is  the  vertical  component,  called  L. 

Drift  is  the  horizontal  component,  called  D. 

The  resolution  of  the  air  pressure  on  an  inclined  surface  into  Lift 
and  Drift,  is  the  fundamental  process  in  the  mechanics  of  the  aeroplane. 

Drift  is  a  drag  or  resistance  to  motion  which  is  overcome  by  the 
thrust  of  the  propeller,  and  at  the  expense  of  which  a  total  inclined 
pressure  is  generated  on  the  aeroplane  surfaces,  the  vertical  compo- 
nent of  which  is  sufficient  to  support  the  weight. 

Since  Drift  is  a  function  of  the  pressure  necessary  to  lift  the  weight, 
it  now  becomes  apparent  why  Drift  was  classified  as  distinct  from  the 
head  resistances  of  the  various  parts  of  a  machine.  The  latter  are 
due  solely  to  their  form  and  the  speed  of  travel,  and  they  exert  no  effect 
on  the  lifting  power  itself. 

Consideration  of  this  resolution  into  Lift  and  Drift,  at  once  in- 
dicates that  the  characteristics  to  be  sought  for  in  a  surface  are  great 
lift  with  a  very  small  drift,  so  that  for  a  minimum  expenditure  of  power  a 
maximum  load  carrying  capacity  is  obtained. 

The  ratio  of  lifting  power,  L,  to  drift  D,  is  a  function  widely  used 
in  considering  the  efficiency  of  surfaces,  and  the  higher  the  value  of 
L/D  the  greater  is  the  weight  that  can  be  carried  per  pound  of  resist- 
ance. 

It  is  well  again  to  emphasize,  that  total  resistance  to  motion  is 
composed  of  two  distinct  items. 

1.  The  air  resistances  of  the  various  parts  of  a  machine,  such  as 
struts,  wires,  wheels,  bodies,  etc. 

2.  Drift  (in  which  is  included  the  head  resistance  and  frictional 
resistance  proper  of  the  wings  alone,  at  the  particular  angle  at  which 
they  are  presented). 


Flat  Planes. 

It  is  necessary  to  draw  a  distinction  between  planes  that  have 
a  flat  cross-section,  and  surfaces  that  have  a  curved  cross-section,  be- 
cause the  variations  of  the  air  pressures  in  magnitude,  position,  and 
dirction  are  quite  distinct. 

Let  P90  represent  the  normal  pressure  on  a  surface  set  at  90°  to 
the  air  stream  and  determined  as  explained  in  Chapter  IV,  pp.  49-50. 
And  let  Pa  represent  the  total  pressure  on  the  surface  when  it  is  set 
at  an  angle  of  incidence  A  to  the  air  stream. 

It  would  be  possible  to  express  the  variation  of  Pa,  with  changes 
in  the  angle  of  incidence  a,  as  a  percentage  of  P90  =  K  S  V2.  This 
would  necessitate  determining  the  ratio  Pa/Pgo,  which  is  called  the 
"ratio  of  inclined  to  normal  pressure."  Then 

Pa   =  Pa/P90KSV2 

where  K  is  chosen  for  the  particular  aspect  ratio  used  (see  p.  50).  This 
is  the  system  ordinarily  employed,  but  for  our  purposes  it  is  consider- 
ably more  convenient  to  return  to  the  conception  of  having  values  of 
K  tabulated  for  each  separate  item.  So  that  we  may  call  Ka  the  value 
of  the  constant  in  the  expression 

Pa   =  Ka  S  V2 

and  proceed  to  investigate  the  values  of  Ka  for  different  angles  of  in- 
cidence, on  the  various  surfaces.  Thus,  if  we  desire  to  determine  the 
total  pressure  on  a  surface  set  at  an  angle  of  incidence,  a  =  6°,  our 
system  of  notation  becomes  quite  clear,  in  stating 

P6  =  K6  S  V2 
Lift  and  Drift. 

It  is  a  fundamental  fact  of  aerodynamics,  capable  of  proof,  that, 
in  flat  planes,  Pa  is  always  perpendicular  to  the  chord.  This  sim- 
plifies the  consideration  of  inclined  pressures  on  flat  planes,  since  at 
any  angle  of  incidence  we  know  the  direction  in  which  the  air  pres- 
sure acts.  Thus,  a  flat  plane,  set  at  an  incidence  of  10°,  is  acted  upon 
by  an  air  force,  the  line  of  action  of  which  is  pointed  80°  above  the 
direction  that  would  be  taken  by  the  normal  pressure. 

This  uniformity  in  the  direction  of  Pa,  with  reference  to  flat  planes, 
enables  us  to  obtain  very  simple  rules  for  finding  the  Lift  and  Drift 
of  flat  sections. 

Obviously  from  the  resolution  of  forces. 

Lift  =  Pa  cosine  a,  =  Ka  S  V2  cos  a 

Drift  =  Pa  sine  a,  =  Ka  S  V2  sin  a 
In  addition,  the  Lift-Drift  ratio,  L/D  =  cotangent  a. 

To  determine  the  magnitude  of  the  forces  on  flat  planes,  there- 
fore, it  is  merely  necessary  to  know  the  appropriate  value  of  Ka,  as 
determined  by  mathematics  or  experiment.  * 

*  In  the  author's  work  "Monoplanes  and  Biplanes,"  many  relations  for  Pa  are 
considered,  in  Chapter  III. 


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The  definitions  for  flat  and  curved  sections,  given  on  p.  57,  are  shown  at  the  top 
of  the  page. 

Curve  2  shows  the  variation  of  Ka,  for  aspects  of  1/3.  1.  3,  and  6. 
Curve  3  shows  the  c.  p.  movement  for  the  various  aspect  ratios. 


61 

The  variations  of  Pa  are  affected  by  Aspect  Ratio,  and  a  very 
remarkable  distinction  between  squares  and  rectangles  in  the  man- 
ner in  which  Pa  varies  as  the  incidence  is  changed  was  discovered  by 
Eiffel.  At  angles  in  the  neighborhood  of  40°,  on  square  planes,  Pa 
was  found  to  have  values  very  much  greater  than  P90. 

Values  of  Ka  for  several  different  aspect  ratios  are  given  in  Curve 
No.  2,  p.  60. 

It  will  be  seen  from  this  graph,  that  an  increase  of  aspect  ratio 
above  1  is  accompanied  by  increases  of  Pa,  at  low  angles.  But  there 
is  a  general  falling  off  of  this  at  15°  to  20°,  as  the  aspect  ratio  is  in- 
creased. For  efficiency,  at  low  angles,  on  flat  planes  it  is  advisable 
therefore,  to  use  the  higher  aspect  ratios. 

In  all  cases,  S  is  the  plan  area  of  the  surface. 

Center  of  Pressure. 

Although  the  direction  and  magnitude  of  forces  on  flat  surfaces 
have  been  considered,  these  forces  are  not  fully  defined  until  determi- 
nations are  obtained  of  their  point  of  application. 

The  nature  of  the  air  reactions  on  a  surface  consists  of  a  series 
of  small  impact  pressures  and  friction  rubs  all  over  the  surface;  but 
their  total  effect  can  be  represented  graphically  by  a  single  force,  in 
the  resultant  direction,  and  applied  at  a  point  about  which  all  pres- 
sures balance. 

This  center  of  balance  of  air  reactions  is  termed  the  "center  of 
pressure,"  and  if  we  draw  thru  it  a  force  proportional  to  Pa,  and  in 
direction  normal  to  the  surface,  we  have  completely  defined  the  air 
reaction  on  that  particular  flat  plane. 

On  flat  surfaces  it  is  indicated  by  Curve  3  that,  as  the  incidence 
is  decreased,  the  center  of  pressure  moves  forward  until  at  0°,  it  is 
very  near  the  front  edge,  and  at  90°  it  is  at  the  center  of  surface. 

The  representation  of  position  of  center  of  pressure,  c.  p.,  as  a 
percentage  of  the  chord,  is  a  convenient  one  that  has  become  quite 
standard. 

Example. 

A  typical  example  of  the  use  of  the  data  given  for  flat  planes  may 
prove  of  interest. 

An  aileron,  flat  in  section,  measuring  2  ft.  chord  by  12  ft.  span 
(aspect  12  -5-  2  =  6),  is  pivoted  4  inches  back  of  the  leading  edge. 
The  aileron  is  moved  to  an  incidence  of  10°  and  the  air  speed  is  60  miles 
per  hour. 

It  is  desired  to  find  the  corrective  force  on  the  balance  of  the  ma- 
chine represented  by  the  lifting  force  of  the  aileron,  at  10°  incidence. 


62 

Lift,  L  =  Ka  S  V2  cos  a. 
From  the  chart  we  find  that  for  a  plane  with  aspect  ratio  of  6, 

Ka  =  .00175, 

and  Cos  10°  =  .985,  S  =  24  sq.  ft.,  V2  =  3600,  so  that 
L  =  .00175  x  24  x  3600  x  .985 

=  149  Ibs. 

In  addition  it  is  desired  to  know  the  moment  of  the  total  pres- 
sure Pa,  about  the  pivot,  at  10°  incidence. 
From  the  graph  it  is  found  that 

c.  p.  position  =  .33  chord  =  .33  x  24 

=  8  inches  from  leading  edge. 

It  follows,  therefore,  that  the  lever  arm  of  the  total  pressure  Pa 
about  the  pivot  is  4  inches. 

Pa  =  Ka  S  V2  =  .00175  x  24  x  3600  =  151  Ibs. 
Therefore,  the  moment  about  the  pivot  is, 
=  151  x  1/3 
=  50  foot  Ibs., 

which  would  enable  the  pounds  pull  on  a  control  mechanism    to    be 
determined,  and  leverages  suitably  arranged. 

Curved  Surfaces. 

Although  the  general  characteristics  of  the  action  of  air  on  flat 
planes  had  been  known  more  or  less  accurately  for  some  time,  the  nature 
of  air  reaction  on  curved  surfaces  was  not  well  appreciated  until  the 
pioneer  work  of  Lilienthal  and  the  Wrights  disclosed  it. 

Lilienthal  discovered  that,  at  low  angles,  surfaces  slightly  cam- 
bered gave  very  much  more  lift  and  less  drift,  than  did  flat  planes, 
at  the  same  incidence,  and  that  the  resultant  total  pressure  was  not 
necessarily  perpendicular  to  the  chord,  as  on  flat  surfaces.  In  fact, 
he  found  that  at  certain  low  angles  of  incidence  the  total  pressure  on 
a  curved  surface  was  leaning  considerably  in  front  of  the  normal  to 
the  chord  line,  which  meant  that  a  smaller  proportion  of  this  total 
pressure  was  drift,  and  a  greater  portion  was  lift. 

It  is  upon  this  discovery  that  the  first  practical  demonstration 
of  the  possibility  of  flight  may  be  said  to  have  originated,  and  suc- 
ceeding generations  are  justified  in  hailing  Otto  Lilienthal,  in  view  of 
his  classic  experiments,  as  the  discoverer  of  modern  flight. 

The  Wrights,  in  their  gliding  experiments,  discovered  that  the 
center  of  pressure  on  an  arched  surface  of  Lilienthal  type,  did  not  change 
its  position,  in  the  same  way  as  the  c.  p.  on  a  flat  plane,  but  that  in- 
stead of  moving  steadily  forward  as  the  incidence  was  diminished  the 
c.  p.  on  the  curved  plane  ceased  to  move  forward  at  about  10°-15°, 
and  retrograded,  moving  rapidly  past  the  center  of  surface,  towards 
the  trailing  edge  as  the  angle  grew  smaller.  This  feature  rendered 
Lilienthal's  measurements  somewhat  inaccurate,  but  the  corrections, 
readily  applied,  were  used  to  make  the  old  results  applicable  to  the 
modern  aeroplane. 


KL  and  KD 

Since  the  total  pressure  on  the  curved  surface,  which  we  will  call 
Pi,  is  not  necessarily  perpendicular  to  the  chord  line  its  resolution 
by  trigonometry  into  Lift  and  Drift  is  not  possible  unless  we  know 
its  inclination  with  respect  to  the  chord  line.  But,  since  this  neces- 
sitates knowing  its  components,  it  becomes,  at  once,  more  convenient 
to  study  curved  surfaces  directly  from  measurements  and  data  on 
Lift  and  Drift. 

This  is  done  throughout  the  study  of  curved  surfaces  and  aero- 
foils (aeroplane  sections),  and  in  the  laboratories  it  is  customary  to 
measure  the  vertical  and  horizontal  forces  on  curved  surfaces.  The 
resultant  of  these  determines  PI,  in  magnitude  and  direction.  But 
since  we  are  rarely  concerned  with  Pi,  where  data  is  already  available 
on  L  and  D,  its  consideration  is  not  so  important. 

To  define  L  and  D,  it  is  most  convenient  to  consider  that 

L   =  KLSV' 

D  =  KD  S  V2 

and*  information  on  curved  surfaces  resolves  itself  into  a  study  of  the 
values  of  KL  and  KD  for  the  various  angles  and  shapes. 

In  this  chapter,  the  simplest  geometrical  curved  sections  only 
are  considered,  as  it  is  desired  merely  to  bring  out  the  main  distinctions 
between  flat  and  cambered  sections. 

The  standard  practice  is  adopted  of  referring  to  the  camber  of 
curved  sections,  as  a  fraction  or  percentage  of  the  chord. 

Forces  on  Cambered  Planes. 

The  resolution  of  P  into  L  and  D  is  fully  indicated  in  the  diagrams 
on  p.  64.  The  angle  of  incidence  of  the  chord  with  the  air  stream 
is  called  i.  But,  since  in  cambered  planes  Pi  is  not  necessarily  normal 
to  the  chord,  it  follows  that  the  angle  between  Pi  and  L  is  not  neces- 
sarily equal  to  the  angle  of  incidence  i,  as  it  is  on  flat  sections.  This 
angle  of  the  resultant  with  the  vertical  is  called  r,  and  from  the  con- 
struction of  the  triangle  of  forces  it  is  apparent  that  tan  r  =  drift/lift, 
and  the  ratio  of  L/D  =  cotangent  r. 

The  nature  and  determination  of  "LilienthaPs  Tangential"  is  not 
considered  necessary  in  this  study,  although  it  is  frequently  dwelt 
upon  in  elementary  aerodynamic  treatises.  * 

Influence  of  Aspect  Ratio. 

The  manner  in  which  a  change  in  Aspect  Ratio  affects  the  forces 
on  circular  arcs  is  shown  in  the  charts  on  p.  64.  It  is  found  that  not 
only  the  magnitude  of  L  and  D,  but  the  movements  of  the  center  of 
pressure  are  influenced  very  much  as  on  flat  sections.  For  a  circu- 

*  See  "Monoplanes  and  Biplanes"  Chapt.  IV,  p.  47. 


(14 


To  find  D  at  any  angle,  divide  values  of  L  by  corresponding  values  of  L/D. 


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Curve  4  shows  coefficients  Lift  and  Ratios  of  L/D  for  changes  in  aspect  ratio, 
of  a  circular  arc. 

Curve  5  shows  the  c.  p.  movements  for  the  same  surfaces. 


65 

lar  arc  in  which  the  camber  is  1/13.5  of  the  chord  (an  average  camber 
for  good  efficiency),  the  values  of  L  and  L/D  steadily  increase  as  the 
aspect  ratio  is  increased  from  1/3  to  6.  The  difference  between  aspects 
of  6  and  9,  however,  is  not  very  marked,  and  it  is  indicated  from  these 
results  that  there  is  not  much  gained  by  increasing  the  aspect  of  an 
aeroplane  wing  above  6.  With  reference  to  the  total  pressure  Pi, 
it  is  found  for  cambered  planes  as  it  was  for  flat  ones,  that  when  the 
aspect  ratio  is  1  (a  square),  Pi  rises  to  a  value  more  than  once  and  a 
half  times  the  normal  pressure  P90  at  an  angle  of  about  40°. 

The  center  of  pressure  chart  shows  that  as  the  aspect  is  increased 
the  reversal  of  movement  at  low  angles  becomes  sharper,  and  the  angle 
at  which  this  reversal  takes  place  falls  from  45°  for  aspect  1/3  to  13° 
for  aspect  6. 

Effect  of  Depth  of  Curvature. 

Alterations  in  the  camber,  i.  e.,  in  the  depth  of  curvature  of  cir- 
cular arcs,  greatly  affect  the  magnitude  of  L  and  L/D,  and  the  move- 
ment of  the  c.  p.  In  the  charts,  on  p.  66  there  are  plotted,  the  curves, 
showing  the  values  of  KL  and  L/D  for  arcs  of  1/27  and  1/7  camber,  with 
an  aspect  of  6,  which  are  to  be  compared  with  the  curve  for  a  1/13.5 
camber,  aspect  6. 

It  will  be  noted  that  the  magnitude  of  L  increases  with  the  in- 
crease of  camber,  but  the  ratio  of  L/D  is  decreased  by  camber  increase 
and  the  point  of  maximum  L/D  varied.  This  indicates  that  deeply 
cambered  sections  would  prove  to  be  inefficient  wings  for  aeroplanes. 

For  the  smaller  camber,  the  c.  p.  movement  is  sharper,  and  the 
reversal  point  further  forward. 

The  Reverse  Curve. 

Sections  of  cambered  surfaces  may,  of  course,  be  other  than  cir- 
cular. Combinations  of  straight  lines  and  circular  arcs,  parabolic 
curves,  spirals,  and  the  like,  have  characteric  pressures  that  differ 
from  each  other,  but  in  so  small  an  amount  that  it  is  hardly  necessary 
to  give  them  separate  consideration  —  excepting  in  so  far  as  they  are 
taken  up  later  in  studies  of  aeroplane  wing  sections. 

The  reversed  curve,  however,  is  a  distinctive  geometrical  section, 
to  which  attention  should  be  given.  These  sections,  as  illustrated  in 
the  diagram,  have  the  important  property  that  the  center  of  pressure 
continues  to  move  forward  as  the  angle  is  decreased,  the  character- 
istic retrograde  movement,  as  found  on  circular  arcs,  being  apparently 
absent.  As  will  be  explained  later,  this  retrograde  movement  tends 
towards  instability,  and  although  the  ratio  of  L/D  and  L  are  very 
greatly  reduced  by  a  reverse  curve,  it  becomes  necessary  to  bear  in 
mind  that  for  aeroplane  wing  sections  the  loss  in  efficiency  may  be 
worth  while,  in  order  to  gain  in  stability. 


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.3          4- 

C.  P.  POSITION 

Curve  6,  shows  the  coefficients  for  Lift,  and  the  values  of  L/D,  for  arcs  of   1/7, 
1/13.5  and  1/27  camber,  and  the  reverse  curve  section. 

Curve  7,  shows  the  c.  p.  movement  for  the  same  sections. 


67 
Aerofoils. 

Only  flat  and  the  simplest  geometrical  sections  have  been  con- 
sidered here.  In  aeroplane  wings,  it  is  necessary  to  have  spars  and 
ribs  of  considerable  depth,  in  order  to  obtain  suitable  strength  and 
rigidity  of  wing.  This  leads  to  sections  of  surfaces  in  which  two  cur- 
vatures must  be  considered  —  the  top  face  and  the  bottom  face. 

An  aeroplane  wing  section  is,  therefore,  distinct  from  geometri- 
cal sections,  and  it  is  customary  to  refer  to  aeroplane  types  of  sur- 
faces as  Aerofoils.  They  are  treated  of  fully  in  Chapter  VII,  since 
a  knowledge  of  their  characteristics,  advantages  and  disadvantages 
enables  the  first  essential  step  in  the  design  of  an  aeroplane  to  be  taken  — 
the  choice  of  a  wing  section  that  will  give  the  weight-lifting,  strength 
and  speed  combination  desired. 

Examples  of  the  application  of  curved  section  pressures  are  taken 
up  in  several  instances  in  the  consideration  of  aerofoils,  and  many  of 
them  are  deduced  from  actual  practice  as  applied  in  well  known  types 
of  aeroplanes. 

Summary. 

I.  In  flat  surfaces. 

The  total  pressure  Pa  is  always  normal  to  the  section,  and 
can  be  resolved  into, 

Lift  =  Pa  Cos  a 
Drift  =  Pa  Sin  a 

The   center   of   pressure  moves   forward   as   the   incidence   is 
decreased. 

II.  In  cambered  surfaces. 

The  total  pressure  Pi  is  not  always  normal  to  chord,  and 
the  pressures  on  a  cambered  plane  are,  therefore,  more  easily  ex- 
pressed as 

Lift  =  KL  S  V2 
Drift  =  KD  S  V2 ' 

The  center  of  pressure,  on  curved  surfaces,  moves  forward  up  to 
a  certain  low  angle  where  it  reverses  and  moves  rapidly  to  the  rear 
(except  for  reverse  curved  surfaces). 


Photographs  of  air  flow,  showing  air  deflections,  obtained  at  the  author's  labor- 
atory, by  introducing  chemical  smoke  into  the  air  stream. 


Normal  Surface  Inclined  Surface 

DIAGRAMS  OF   AIR   FLOW 


Photographs  of  air  flowing  from  left  to  right,  on  flat  surfaces,  made  at  the  Kout- 
chino  laboratory.  Note  the  general  deflection  of  the  air  stream,  in  the  lower  right 
hand  photo,  where  the  plane  is  at  a  low  angle  of  incidence. 


CHAPTER  VI. 
AERODYNAMIC  THEORY. 

Although  it  is  not  so  essential  to  consider  the  theoretical  deriva- 
tion of  formulae  for  air  resistances  in  a  work  of  this  kind,  a  certain 
interest  is  attached  to  the  application  of  the  more  recent  experiments 
on  the  flow  of  air  streams  to  the  older  conception  of  the  mechanics  of 
the  air. 

An  outstanding  experimental  fact  in  air  stream  photographic 
studies,  that  vitiates  many  established  aerodynamic  derivations,  is 
that  the  air  stream,  when  it  impinges  against  a  normal  surface,  divides 
to  pass  around  the  edges,  and  in  doing  so  actually  imprisons  a  cushion 
of  dead  air  against  the  surface,  a  phenomenon  constantly  met  with 
in  wind  effects  on  moving  vehicles,  etc.  Furthermore,  in  dividing, 
the  air  stream  acts  as  if  separated  by  two  physical  surfaces  and  takes 
a  deflection  of  about  45°,  instead  of  being  turned  thru  an  angle  of  90°, 
as  assumed  in  older  hypotheses.  Many  photographs  of  air  flow,  in- 
cluding several  taken  in  the  writer's  laboratory  in  New  York,  confirm 
this. 

The  mechanics  of  air  flow  on  normal  surfaces,  if  the  air  is  con- 
sidered as  deflected  at  90°,  may  be  stated  as  follows: 

Moving  air  develops  a  pressure  equal  to  its  momentum,  expend- 
ing its  entire  energy  by  impact.  Momentum  =  mass  X  velocity.  The 
mass  of  air  =  W/g,  where  W  is  its  total  weight,  and  equal  to  the  unit 
weight  or  density  of  air  w,  X  the  volume  of  air  moved,  which  is  equal 
to  S  V,  where  S  is  the  surface  in  sq.  ft.  and  V  the  velocity  in  ft.  per 
sec. 

Supplying  appropriate  values  we  get  for  the  total  resistance, 

P90  =  w/gSV2 
=  .0054  S  V2 

which  is  a  formula  of  the  form  P  =  K  S  V2  in  which  K  =  .0054. 

This  value  of  K  we  know  by  experiment  is  quite  incorrect,  and 
it  follows  that  we  must  consider  the  derived  result  worthless. 

If,  however,  we  start  with  the  more  correct  hypothesis,  that  the 
air  is  deflected  at  about  45°  (depending  possibly  on  friction  effects) 
instead  of  90,°  a  derivation  of  this  form  would  result. 


70 

Let  A  B,  in  diagram  p.  68,  represent  the  surface  of  area  S,  in  an 
air  stream  of  velocity  V.  Let  ABC  represent  the  imprisoned  cushion 
of  air,  and  A  C  and  C  B  the  surfaces  of  air  along  which  the  deflected 
stream  flows. 

Let  s'  and  s"  represent  the  normal  projections  of  A  C  and  B  C. 

The  total  energy  of  the  air  stream,  before  deflection,  is  represented 
by  N'  +  N",  where  N'  =  w/g  s'  V2  and  N"  =  w/g  s"  V2. 

The  resolution  of  N'  into  D'  and  F',  indicates  the  probable  state 
of  affairs  along  the  "deflection"  plane  A  C.  The  force  D'  is  parallel 
to  and  vanishes  with  the  deflected  air  stream,  and  in  fact  represents 
the  stream's  energy.  The  force  F',  however,  is  perpendicular  to  the 
plane  A  C,  and  since  its  action  is  directed  towards  the  surface  it  can 
be  resolved  into  a  force  P',  normal  to  the  surface,  and  in  the  same  di- 
rection as  P90  and  a  force  R',  equal  and  opposite  to  R",  which  is  entirely 
used  up  in  compressing  the  air  cushion.  While  a  greater  part  of  the 
energy  of  the  stream  D',  D",  goes  away  with  it,  a  portion  P',  P",  of  the 
stream's  force  is  "deposited"  on  the  surface, 

P90  =  P'  +  P". 
Analysis  shows  that 

N'  =  w/g  S'  V2  and  N"  =  w/g  S"  V2. 

AC  =  BC  =  S  sin  45°  and  s'  =  AC  sin  45°  =  S  sin2  45°  =  S/2  =  s' 
F'  =  N'  sin  45°,  F"  =  N"  sin  45°,  and  P'  =  F'  sin  45°,  P"  =  F"  sin  45C 

Recalling  that  sin2  45°  =  0.5 

P'  =  w/g  S  0.5  V2  sin2  45°  and  P"  =  w/g  S  0.5  V2  sin2  45° 
Hence  using  sea  level  and  mean  temperature,  conditions,  for  w/g, 
P90  =  w/g  S  V2  sin2  45° 
=  .0054  S  V2  0.5 
=  .0027  S  V2 

which  is  so  nearly  in  accord  with  experimental  results,  K  =  .003  as 
at  once  to  lead  to  an  appreciation  of  the  value  of  considering  the  "air 
cushion,  45°  deflection"  characteristics  of  air  flow,  in  any  study  of  air 
pressures. 

The  experimental  results  on  the  pressures  experienced  by  inclined 
planes  show  that  at  angles  above  45°  the  change  in  inclination  does- 
not  greatly  affect  the  pressures.  The  division  of  the  air  in  front  of 
a  plane  inclined  at  angles  greater  than  45°  is  of  the  same  character! 
as  in  normal  surfaces,  and  a  similar  theory  when  applied  shows  that' 
the  pressure  remains  constant  from  90°  to  45°. 


71 

Inclined   Surfaces 

Referring  to  the  diagram,  p.  68,  the  mathematics  of  this  develops  as 
follows : 

AC  =  S  Sin  (a  -  45°)  and  BC  =  S  Cos  (a  -  45°) 

.'.  S'  =  S  Sin  (a  -  45°)  Sin  45°  and  S"  =  S  Cos  (a  -  45°)  Sin  45°. 

The  normal  pressures  on  the  projected  areas,  S'  and  S'',  may  be 
considered  as  the  energy  of  the  air  stream,  a  proportion  of  which  is  ex- 
pended on  surface  AB  as  P. 

Calling  N'  and  N"  these  pressures,  we  have  N'  =  w/g  S'  V2  and 
N"  =  w/g  S"  V2. 

The  force  triangles  show  that,  P  =  P'  +  P"  and  that  P'  =  N'  Sin 
(a  -  45°)  Sin  45°  and  P"  =  N"  Cos  (a  -  45°)  Sin  45°. 

Supplying  the  values  of  S',  S"  and  of  the  resulting  N'  and  N",  we 
get  P  =  w/g  SV2  Sin2  45°  Sin2  (a  -  45°)  +  w/g  SV2  Sin2  45°  Cos2  (a  -  45°). 

Since  Sin2  +  Cos2  =  1,  and  Sin2  45°  =  0.5,  and  supplying  values 
of  w  and  g  there  is  obtained, 

P  =  .0027  SV2 

From  35°  to  45°,  the  inclined  flat  surfaces  there  is  a  region  of  un- 
steady flow,  in  which  for  squares  the  pressures  become  much  greater 
than  the  normal. 

Below  these  angles  the  air  flow  ceases  to  divide  along  deflection 
planes  in  front  of  the  surface,  and  all  of  the  air  passes  under  the  sur- 
face. In  this  case  the  pressure  would  be  proportional  to  the  sine  of 
the  angle  of  incidence,  as  outlined  above,  in  forces  F'  F"  and  the 
formula 

Pa  =  K  S  V2  sin  a 
is  found  closely  to  agree  with  practice. 

This  theory,  first  proposed  by  the  writer  some  time  ago,  is  as  rigid 
as  any  resolution  into  Lift  and  Drift.  The  hypotheses  may  be  briefly 
summarized  as  the  consideration  of  the  division  of  the  air  along  two 
deflection  planes,  which  act  like  a  surface  on  the  air,  and  cause  the 
energy  of  the  air  to  be  divided  up  into  a  force  parallel  to  the  deflected 
stream,  which  goes  along  with  it,  and  a  force  normal  to  the  layer  of  air, 
along  which  deflection  takes  place,  which  in  turn  is  composed  of  a  force 
compressing  the  air  cushion  and  a  force  actually  causing  the  resistance 
of  the  surface. 

Proper  consideration  of  air  deflection  is  capable  of  determining 
c.  p.  position  (by  intersection  of  the  deflection  planes)  and  when  ap- 
plied to  curved  surfaces,  should  give  most  interesting  results.  And, 
it  would  seem,  that  additional  measurements  by  the  laboratories,  on 
the  angles  of  the  deflected  currents,  would  make  the  data  on  surfaces 
more  complete. 

It  is  seen,  therefore,  that  derivations  based  on  a  more  accurate 
hypothesis  of  air  flow,  give  much  more  satisfactory  results.  In  a  work 
dealing  so  largely  with  practice  it  is  important  to  point  out  that  unless 
the  hypothesis  of  the  physical  air  flow  used  in  any  theory  is  correct  it 


72 

is  far  more  practical  to  rely  on  observations  and  abandon  formulae. 
The  theory  of  propellers,  and  its  lack  of  agreement  with  practice,  is  a 
field  in  which  there  is  a  most  pressing  need  of  a  satisfactory  basis  for 
theoretical  determinations. 

The   "Absolute"    System  of  Units. 

It  has  been  indicated  that  K,  in  the  formula  P  =  K  S  V2  is  a  func- 
tion of  w/g  of  air.  For  any  body,  if  we  introduce  another  constant 
C,  we  may  write  the  air  resistance  formula  in  terms  of  density  of  air  w, 
and  acceleration  of  gravity  g,  as 

P  =  C  w/g  S  V2 

Since  the  units  in  which  P  is  expressed  depend  on  the  units  used 
for  w  and  g,  S  and  V,  it  follows  that  C  is  a  number  independent  of  the 
system  of  units  employed.  For  this  reason  it  is  called  the  absolute 
coefficient,  and  its  value  is  the  same  whether  P  is  expressed  in  Ibs.  or 
grams,  providing  the  expression  of  w/g  is  made  in  the  proper  units. 

The  absolute  system  is  used  by  the  Goettingen  and  the  N.  P.  L. 
(British)  Laboratories.  It  is  an  inconvenient  system  for  practical 
field  use,  but  for  the  international  comparison  of  scientific  results  it 
is  admirably  adapted. 

The  system  used  in  this  work  is 

P  (pounds)  =  K  S  (sq.  ft.)  V2  (miles  per  hour). 

Therefore,  to  translate  results  in  the  absolute  system  to  these 
units  the  "absolute"  values  must  be  divided  by  196. 

"Absolute"  values  x  .0051  =  "m.  p.  h.,  sq.  ft."  units 

The    Metric   System. 

The  Eiffel  results  are  expressed  in  metric  units, 

P  (kilograms)  =  K  S  (sq.  meter)  V2  (met.  per  sec.) 
so  that  Metric  values  x  8  =  "absolute"  values,  and 

Metric  values  x  .041  =  "m.  p.  h.,  sq.  ft."  units. 
Thus  for  K  =  .0033,  we  would  have  in  "absolute"  units 

P  =  .64  w/g  S  V2 
and  in  metric  units,  P  =  .08  S  V2. 

In  the  consideration  of  these  conversion  factors,  atmospheric 
pressure  at  sea  level  and  ordinary  temperature  conditions  are  assumed. 

Summary. 

The  greater  part  of  this  chapter  is  presented  for-  reference,  but  the 
different  systems  of  units  and  the  conversion  factors  are  of  import- 
ance, and  should  be  understood,  and  borne  in  mind. 

The  theory  of  air  pressure  presented,  is  purposely  not  expressed 
in  terms  of  the  usual  theorems  and  laws  of  fluid  dynamics,  since  it  is 
iesired  to  emphasize,  merely,  the  importance  of  continually  bearing 
in  mind,  that  the  resolution  of  air  pressures,  along  the  directions  in 
which  they  act,  is  the  correct  fundamental  conception  of  aerodynamics. 


CHAPTER   VII. 
CHARACTERISTICS   OF  AEROFOILS. 


The  manner  in  which  air  pressures  vary  on  flat  and  cambered 
surfaces  has  been  considered  fully  enough  to  enable  us  to  proceed  with 
the  study  of  aeroplane  wings  themselves.  As  already  indicated,  the 
necessity  of  having  spars  and  ribs  of  considerable  depth  for  the  rigidity 
of  an  aeroplane  surface,  makes  it  necessary  to  use  a  section  of  a  cer- 
tain thickness,  and  consequently  two  curvatures  —  the  top  face  and  the 
bottom  face  —  must  be  taken  into  consideration. 

Aeroplane  wing  sections  are  ordinarily  referred  to  as  aerofoils, 
and  the  study  of  the  various  aerofoils  and  their  characteristics  is  of 
very  real  importance.  While  a  good  deal  of  the  data  on  air  pressures 
has  been  given  by  way  of  explanation  and  general  information,  the 
wing  characteristics  referred  to  here  are  of  the  greatest .  practical  sig- 
nificance, and  are  every  day  being  put  to  use  and  verified,  on  the  avia- 
tion fields  of  the  military  world.  The  connection  between  the  character- 
istics of  a  wing  section  and  the  operation  of  a  great  war  would  seem 
remote,  but  when  it  is  appreciated  that  superior  speed  and  climbing 
ability  enables  a  hostile  aeroplane  to  gather  information  quickly  and 
escape  from  attack  and  pursuit,  primarily  because  of  the  greater  effi- 
ciency of  its  wing  section,  the  importance  of  this  study  becomes  ap- 
parent. 

The  development  of  wing  sections  has  been  along  several  lines. 
Originally  geometrical  sections  were  made  thick  enough  to  give  room 
for  spars  and  then  rounded  at  the  edges.  Other  pioneers,  after  de- 
ciding on  the  size  of  spar  and  thickness  required,  adopted  a  certain 
camber  for  the  mean  center  line,  and  then  proceeded  to  fill  out  a  sec- 
tion that  would  streamline  the  spars.  Still  other  investigators  adopted 
parabolic  and  circular  curve  combinations,  crescent  shapes,  etc.,  and 
finally  the  great  laboratories  took  up  the  matter  and  systematized  its 
study.  The  Eiffel,  N.  P.  L.,  and  Goettingen  results  are  complete 
enough  now  to  give  a  very  firm  basis  for  aeroplane  design,  and  to  en- 
able the  effects  of  any  changes  in  aerofoils  to  be  quite  accurately  an- 
ticipated. 

In  general  the  features  of  an  aeroplane  wing  that  may  be  varied 
are: 

1.  Shape  of  Section,  curvature,  thickness,  etc. . 

2.  Shape  in  Plan,  contour,  aspect  ratio. 


74 

In  addition,  the  manipulations  of  the  wing  by  warping  or  moving 
flap  sections  that  are  connected  to  it,  modify  the  pressures,  and  there 
are  further  modifications  of  the  air  forces  when  the  proximity  of  some 
other  wing  or  body  affects  the  air  flow  and  interferes  with  the  paths 
of  the  streamlines.  Mutual  interference  of  surfaces  with  each  other 
is  a  formidable  study,  and  is  given  special  consideration. 

The  nature  of  air  pressure  on  aerofoils  is  revealed  by  air-stream 
photographs,  the  most  striking  feature  being  the  manner  in  which 
the  rounded  nose  of  the  aerofoil  in  deflecting  the  air  stream,  causes 
the  air  some  distance  ahead  to  take  a  curvilinear  path  up  to  the  aero- 
foil. This  influence,  frequently  called  the  "phenomenon  of  the  dip- 
ping front  edge,"  is  very  pronounced  for  some  aerofoils,  and  when 
generating  an  upward  stream  of  this  kind  an  aerofoil  is  virtually  rid- 
ing on  the  crest  of  a  wave.  An  explanation  is  found  here  for  the  greater 
lift  and  less  drift  of  aerofoils,  and  the  section  that  generates  the  most 
pronounced  wave  with  the  least  break  in  the  flow  is  naturally  the  most 
efficient. 

Another  feature  that  it  becomes  more  necessary  to  consider  now, 
in  view  of  the  separate  nature  of  the  top  face,  and  the  bottom  face, 
is  that  the  total  air  reaction,  resulting  in  the  forces  on  the  aerofoil, 
consists  of  pressure,  both  positive  and  negative.  Positive  pressure 
is  a  compressive  action,  while  negative  pressure  is  a  suction.  In  pre- 
vious considerations  of  air  resistances,  it  has  been  unnecessary  to  draw 
this  distinction,  since  we  were  interested,  merely,  in  the  total  effect 
of  the  air  reaction. 

Lift  by  Suction  on  Top  Face. 

Careful  studies  of  the  distribution  of  pressure  over  the  surface 
of  typical  aerofoils  made  by  the  great  laboratories,  have  shown  that 
the  actual  effect  of  the  air  flow  at  the  usual  flying  angles  is  not  only 
to  generate  a  pressure  (compression  of  air)  on  the  lower  face  of  the 
inclined  surface,  but  also  to  cause  a  great  suction  on  the  upper  face. 
Furthermore,  measurements  show  that  the  value  of  this  suction  in 
pounds  force  is  about  three-quarters  of  the  total  air  force  on  the  aero- 
foil. In  other  words,  the  action  of  the  air  flowing  past  an  aeroplane 
wing,  primarily  causes  a  partial  vacuum  on  the  top  face,  which  tends 
to  draw  the  surface  up  by  suction.  As  long  as  this  wave  form  suction 
type  of  flow  continues,  the  surface  is  in  its  most  favorable  attitude  for 
efficiency,  but  at  higher  angles  than  ordinarily  employed  in  flying  this 
type  of  flow  breaks  down,  and  a  disruption  of  the  streamlines  follows, 
evidenced  on  the  surface,  by  a  great  increase  in  resistance  and  fall 
in  lift.  The  angle  at  which  this  change  of  flow  occurs  is  called  the 
critical  angle. 

While  the  distribution  of  pressure  across  the  wing's  chord  is  of 
the  form  indicated  on  p.  78,  there  is  also  good  reason  for  investigat- 
ing the  manner  in  which  the  pressures  on  a  wing  vary  from  the  center 


7,5 

across  the  span  to  the  tips.  As  the  tip  is  approached  the  pressures 
reduce  and  the  point  of  highest  suction  passes  from  the  leading  edge 
towards  the  trailing  edge.  The  drift  of  the  wing  tips  is  found  to  in- 
crease and  to  be  accompanied  by  a  fall  in  L/D,  as  the  tip  is  approached. 
The  type  of  flow  that  produces  the  best  L/D  is  found  at  the  center  of 
the  wing,  where  the  streamlines  pass  directly  from  front  to  rear.  As 
the  tips  are  approached,  however,  the  streams  of  air  begin  to  flow  off 
sideways,  endeavoring  to  escape  out  at  the  sides.  Obviously,  the 
higher  the  aspect  ratio,  the  less  in  proportion  is  this  sideways  escape 
of  air,  and  therefore  the  better  the  L/D. 

General   Characteristics. 

Although  the  pressures,  on  the  various  sections  differ  consider- 
ably from  each  other,  there  are  certain  characteristic  features  that  are 
common  to  the  majority  of  the  aerofoils. 

At  0°  incidence  there  is  usually  a  certain  lift  A,  and  at  a  negative 
angle,  anywhere  from  -2°  to  —9°,  there  is  a  point  of  no  lift,  H  (see  p. 
78).  The  manner  in  which  the  Lift  and  L/D  curves  are  plotted,  on 
a  basis  of  angles,  is  the  same  as  in  Chapter  V,  the  Lifts  being  defined 
by  values  of  KL  in  the  formula,  Lift  =  KL  S  V2.  From  A  to  C,  on 
the  Lift  curve,  is  more  or  less  of  a  straight  line,  the  curve  bending  over 
at  C,  which  point  is  called  the  point  of  maximum  lift.  From  C  to  D, 
instead  of  continuing  to  increase,  a  critical  state  of  flow  has  been  reached, 
where  further  incidence  increase  is  accompanied  by  a  drop  in  the  Lift. 
This  is  an  interesting  portion  of  the  curve,  and  we  will  again  have  to 
refer  to  it  when  we  take  up  the  control  of  the  aeroplane  in  this  reversed 
pressure  region.  Where  lift  decreases  in  this  way,  it  may  be  stated 
briefly,  that  the  controls  on  an  aeroplane  would  have  to  be  reversed 
for  flying  in  this  region.  To  go  up,  it  would  be  necessary  to  reduce 
the  angle  of  incidence,  and  to  descend,  the  elevator  would  have  to  be 
pulled  back  so  as  to  increase  the  angle. 

On  the  L/D  curve  from  the  point  of  F  at  0°,  the  value  of  L/D 
increases  to  a  maximum  E,  corresponding  to  a  Lift  of  value  B.  From 
E  to  G,  the  L/D  ratio  again  falls  off.  The  ordinary  regions  of  flight 
are  limited  to  the  peak  region  of  the  L/D  curve. 

In  the  study  of  the  aeroplane,  as  a  unit,  taken  up  later,  consid- 
eration will  be  given  to  the  important  relations  that  the  maximum 
and  minimum  points  of  the  L  and  L/D  curves  bear  to  each  other. 

Having,  in  a  general  way,  considered  the  nature  of  air  reaction  on 
aerofoils,  we  may  proceed  with  a  study  of  the  effect  of  alterations  in 
shape  and  plan  form  and  interference.  The  values  of  KL  and  L/D 
in  the  curves,  refer  to  the  combined  action  of  whatever  compression 
or  suction  is  generated,  unless  otherwise  noted.  It  is  also  well  to  re- 
call that  L/D  represents  the  ratio  of  the  Lift  force  obtained  from  an 
aerofoil  at  the  expense  of  the  Drift  D,  a  resistance  that  must  be  over- 


76 

come.     "Efficiency"  refers  to  L/D,  and  is  higher,  the  greater  the  Lift 
obtained  for  resistance  overcome. 

ALTERATION   IN   SHAPE  OF  SECTION. 

1.  Camber  of  Upper  Face. 

Increasing  the  camber  of  the  top  face  from  1/40  to  1/6,  on  a  form 
with  a  flat  under  face,  shows  that  the  maximum  lift  increases  up  to 
a  camber  1/15  and  then  decreases.  The  ratio  of  L/D  steadily  im- 
proves up  to  a  camber  of  1/20.  This  camber  appears  the  most  efficient, 
as  deeper  cambers  show  a  steady  decrease  in  values  of  L/D. 

On  an  aerofoil,  having  the  under  face  arched  considerably,  when 
the  camber  of  the  upper  surface  is  increased  above  1/15,  the  Lift  hardly 
varies,  while  the  drift  steadily  increases  with  camber  increase.  For 
very  thick  sections,  however,  just  as  in  spheres  and  cylinders,  there 
is  a  critical  flow,  which,  due  to  increases  in  speed,  tends  to  smooth  out 
and  reduce  resistance. 

2.  Camber  of  Lower  Face. 

Increasing  the  camber  of  the  lower  face,  for  a  fixed  upper  face, 
shows  that  L/D  does  not  vary  very  much,  and  that  L  increases  ap- 
preciably with  camber  increase.  Since  the  depth  of  spar  is  very  greatly 
enhanced  by  keeping  a  flat  underside,  there  is  every  reason  for  con- 
sidering rather  flat  under  surfaces  as  advantageous.  The  increased 
depth  of  spar  reduces  the  weight  of  framework  in  the  wing  necessary 
for  a  given  strength,  and  would  about  compensate  for  the  lift  increase 
obtained  by  camber  of  the  lower  face. 

The  upper  face,  furnishes  most  of  the  lift  and  variations  of  lower 
face  have  very  little  effect  on  the  upper  side. 

3.  Thickness  and  Depth  at  Rear. 

By  keeping  the  same  mean  curve  of  a  section,  and  adding  to  the 
top  and  bottom  faces  at  the  same  time,  the  drifts  are  found  to  remain 
about  the  same,  and  a  decrease  in  lift  is  found  as  the  section  becomes 
more  and  more  a  streamline  body  —  due  to  the  progressive  bulging  out 
of  the  section,  both  top  and  bottom. 

For  any  particular  curve  a  thickening  of  the  rear  alone  to  permit 
of  a  deeper  rear  spar,  shows  a  decrease  in  L/D  with  increase  in  thick- 
ness and  a  slight  decrease  in  Lift,  but  this  is  not  so  very  marked,  and 
sections  can  be  deepened  at  the  rear  with  ease,  thus  permitting  of  hav- 
ing the  front  and  rear  spars  of  the  same  depth. 

4.  Bluntness  and  Streamlining  of  Nose. 

Substituting  a  blunt  for  a  sharp  leading  edge,  causes  the  ratio  of 
L/D  to  fall  off,  but  since  L  remains  about  the  same  there  is  indicated 
a  pronounced  increase  in  D.  Bluntness  of  the  nose  may,  therefore, 


77 

be  considered  a  disadvantage.  Pointing  the  nose  of  an  aerofoil,  to  a 
streamline  shape,  designed  to  divide  the  air  easier,  often  called  a  "Phillip's 
Entry,"  is  frequently  used.  It  is  advantageous  in  decreasing  the 
drift  slightly  at  high  speeds  and  low  angles,  but  otherwise  has  little 
effect. 


5.     Changing  Position  of  Maximum  Ordinate. 

The  fraction  of  the  chord  at  which  the  camber  is  the  greatest  is 
termed  the  position  of  maximum  ordinate.  It  can  readily  be  varied 
on  aerofoils,  and  it  is  found  that  L/D  increases  as  it  is  moved  from  the 
center  of  the  surface,  or  .5  chord,  towards  the  leading  edge  until  it 
reaches  the  position  of  1/3  chord,  when  further  movement  forward 
greatly  reduces  the  efficiency  of  the  section.  The  Lift  is  not  affected 
very  much  at  low  angles  by  changes  in  the  position  of  the  maximum 
ordinate,  but  at  high  angles  the  lift  of  the  section  falls  off  when  the 
greatest  camber  is  at  a  point  in  front  of  1/3  chord. 


6.  Reverse  Curvature. 

Reversing  the  curve  of  either  face  of  an  aerofoil,  has  a  pronounced 
effect  on  the  c.  p.  movement.  The  lower  face  of  a  deeply  cambered 
aerofoil  is  readily  made  to  reverse  at  the  rear  and  meet  the  upper  face. 
This  is  often  done,  and  is  distinctly  beneficial. 

More  pronounced  reverse  curves  in  which  both  faces,  at  the  rear, 
are  turned  up,  have  a  very  great  influence  on  the  air  pressures.  The 
advantageous  feature  of  having  a  stationary  center  of  pressure  posi- 
tion for  the  various  angles,  is  obtained  by  raising  the  trailing  edge 
about  .037  of  chord  —  the  curve  starting  from  a  point  about  .2  of  chord 
from  the  trailing  edge.  But  in  doing  this  the  maximum  lift  is  reduced, 
and  the  range  of  lift  restricted.  There  is  also  a  speedy  decrease  in  L/D, 
and,  in  general,  this  change  leads  to  inefficiency,  reducing  lift  by  about 
25%  and  max.  L/D  by  about  15%. 

7.  Warping  the  Aerofoil. 

"Warping"  an  aerofoil  consists  in  twisting  it  in  such  a  way  as 
to  have  the  various  sections  presented  to  the  air  at  uniformly  vary- 
ing angles  of  incidence.  The  section  of  wing  remains  constant,  and 
since  its  characteristics  for  varying  angles  are  known,  the  amount  of 
pressure  at  the  different  sections  could  be  found.  In  the  ordinary 
range  of  warp  in  practice,  on  aeroplane  wings  where  one  side  is  moved 
up  and  the  other  down  equally  at  the  same  time,  the  Lift  remains  the 
same,  as  does  also  the  position  of  the  mean  center  of  pressure,  and 
tests  show  that  computations  on  a  basis  of  applying  the  ordinary  data 
for  the  section  to  the  different  regions  at  their  various  angles 
gives  correct  results. 


78 

ASPECT    RATIO   TABLE 

Values  tabulated  are  the  ratios  of  L  and  L/D  at  given  aspect  to  values  for  an 
aspect  of  6. 

ANGLES 


ASPECTS 

3° 

6° 

9° 

L 

L/D 

L 

L/D 

L 

L/D 

2 

.60 

.47 

.62 

.54 

.60 

.55 

3 

.70 

.58 

.73 

.64 

.78 

.72 

4 

.84 

.73 

.85 

.77 

.90 

.83 

5 

.94 

.86 

.95 

.90 

.96 

.92 

6 

1.00 

1.00 

1.00 

1.00 

1.00 

1.00 

7 

1.05 

1.06 

1.04 

1.10 

1.04 

1.09 

8 

1.08 

1.09 

1.08 

1.16 

1.08 

1.15 

1 

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PRESSURE   DISTRIBUTION    -  TYPICAL   CURVES  -  AND   DEFINITIONS 


79 

ALTERATION    IN   PLAN   FORM. 
Shape  of  Plane. 

Cutting  away  the  trailing  edge  at  the  tips,  and  rounding  off  the 
ends  of  the  plane,  is  often  resorted  to  for  reasons  of  construction  and 
appearance.  It  is  found  that  this  does  not  appreciably  affect  the 
pressures,  and  cutting  away  the  tips  slightly  reduces  the  weight  of 
wing.  On  the  other  hand,  it  is  found  that  raking  the  ends  of  a  plane  or 
that  the  trailing  edge  is  of  greater  span  than  the  leading  edge,  does 
appreciably  affect  the  pressures,  the  Drift  being  considerably  reduced 
and  the  ratio  of  L/D  improved.  The  gain  in  efficiency  is  due  undoubt- 
edly to  a  better  utilization  of  the  sideways  flow  of  air,  in  escaping  past 
the  edges.  For  the  best  results,  where  consideration  is  given  to  the 
strength  of  the  wing,  the  ends  should  be  raked  at  angles  of  20°  to  30°. 

Aspect  Ratio. 

The  influence  of  aspect  ratio,  on  the  pressures  experienced  by 
aerofoils  is,  of  course,  quite  similar  to  its  effect  upon  geometrical  sec- 
tions. It  becomes  quite  important  for  us  to  consider  this,  with  refer- 
ence to  aerofoils,  in  greater  detail,  since  aeroplanes  vary  considerably 
in  aspect.  The  "aspect"  of  an  aeroplane  is  always  considered  as  its 
total  span  -f-  by  the  chord  of  wings,  the  wings  not  being  considered 
separately  from  their  attachment  to  the  body. 

Although  Pa  on  flat  planes  is  affected  by  aspect,  the  ratio  of  L/D 
is  not  so  affected,  since  it  is  always  a  function  of  angle  of  incidence  a, 
as  outlined  in  Chapter  V.  But  on  aerofoils,  not  only  does  D  vary,  but 
there  is  a  very  pronounced  change  in  L/D. 

As  the  aspect  ratio  is  increased  from  2  to  8,  the  usual  limits  used 
in  practice,  the  maximum  lift  coefficient  remains  at  about  the  same 
value,  but  it  occurs  at  smaller  angles  of  incidence  as  the  aspect  ratio  is 
increased. 

The  most  marked  change,  due  to  aspect  ratio  variation,  is  in  the 
value  of  L/D.  This  is  found  to  be  due  mainly  to  an  increase  in  the 
Drift,  for  the  smaller  aspects. 

The  average  aeroplane,  has  an  aspect  of  6,  which  it  is  found  is  a 
good  value,  but  an  increase  up  to  8  and  9,  is  justifiable,  since  the  limit 
in  improvement  of  efficiency  becomes  pronounced  only  for  these  higher 
aspects.  For  very  flat  sections  of  camber  1/30,  or  thereabouts,  the 
ratio  of  L/D  is  found  to  decrease  at  very  low  angles,  when  the  aspect 
is  increased  above  5.  At  higher  angles,  higher  aspects  give  better 
efficiency  as  in  deeper  cambered  planes,  but  it  would  appear  that  for 
the  flatter  sections,  used  at  low  angles  and  very  high  speed,  on  the 
small  fast  scouting  aeroplanes,  there  is  justification  for  limiting  the 
aspect  ratio  to  about  5  —  a  feature  that  is  structurally  very  advan- 
tageous. 


The  most  convenient  way  to  present* data  on  aspect  ratio  has  been 
a  matter  of  question,  and  a  system  is  adopted  here  which,  it  would  seem, 
is  the  most  practical  for  the  use  of  the  engineer  and  the  aeroplane  user. 
The  data  for  wing  sections  given,  is  in  every  case,  excepting  where 
otherwise  noted,  reduced  and  corrected  to  correspond  to  an  aspect 
ratio  of  6.  In  addition,  accompanying  this,  is  a  table  which  gives 
the  factor  by  which  to  multiply  values  for  any  other  aspect  ratios  from 
2  to  8,  the  aspect  ratio  of  6,  being  considered  as  unity  (see  p.  78). 

For  example,  at  3°,  the  L/D  of  N°  36  Eiffel  surface  is  found  from 
the  graph  to  be  14.7  for  an  aspect  of  6,  and  the  corresponding  lift  co- 
efficient, KL  is  .0014.  It  is  desired  to  know  what  the  values  would  be 
for  an  aspect  of  4.  From  the  table,  we  find  that  L/D  will  be  73% 
of  the  value  of  14.7,  which  is  10.7,  and  the  value  of  KL  will  be  84%  of 
.0014,  which  is  .00118.  If  it  is  desired  to  know  the  values  for  angles 
between  3°  and  6°,  it  is  easiest  to  plot  the  values  of  3°,  6°,  9°  on  the 
chart,  and  draw  thru  them  curves  entirely  symmetrical  and  of  the 
same  character  as  the  ones  for  the  aspect  of  6. 

For  field  use,  the  table  is  put  in  a  novel  form,  but  one  which  it  is 
thought  is  far  handier  than  any  hitherto  published.  The  combined  re- 
sults of  all  the  laboratories  were  given  consideration  in  deriving  the 
values  given.  ' 

Effects  of  Speed  and  Scale. 

In  stepping  from  model  tests  to  full-sized  machines,  the  best  ap- 
proximation at  present  made  appears  to  work  out  quite  well  in  practice. 

Lift  values,  of  coefficient  KL,  are  applied  directly  without  any  cor- 
rection. 

Friction  effects  on  Drift  cause  it  to  decrease  with  increase  of  speed, 
and,  therefore,  at  speeds  higher  than  the  wind  tunnel  speeds,  the  value 
of  L/D  will  be  greater.  The  Eiffel  results,  however,  were  obtained  in 
winds  of  50  to  70  miles  per  hour  and  require  no  correction,  and  in  or- 
der to  bring  the  other  results  presented  in  accord,  correction  for  speed 
has  been  made  wherever  necessary.  The  values  given,  therefore,  may 
be  applied  without  further  correction  to  full-sized  machines,  at  ordinary 
speeds,  by  supplying  the  values  of  S  and  V2. 

Pressures  are,  of  course,  functions  of  V2  of  the  aeroplane,  and  the 
corrections  mentioned  apply  only  to  the  values  of  KL  and  L/D  tabu- 
lated. Pressures  are  also  functions  of  areas,  and  therefore  vary  as  the 
scale  of  the  model  squared.  In  the  wind  tunnels  pressures  are  meas- 
ured in  pounds,  let  us  say,  and  a  particular  pressure  on  an  aeroplane 
model  to  1/10  scale  is  found  to  be  1  pound,  in  a  wind  of  30  miles  per 
hour.  It  is  desired  to  know  what  the  force  on  the  aeroplane  will  be  at 
60  miles  per  hour.  The  observed  value  must  be  multiplied  by 
60 2  3600 

-  X  102,or x  100  =  400  pounds. 

302  900 


81 

Typical   Sections   of  Aerofoils. 

> 

Twelve  aerofoil  sections  that  represent  a  wide  variety  of  actual 
practice  are  tabulated.  The  sections  are  drawn  out  all  to  .the  same 
scale,  and  the  center  of  pressure  graph  is  drawn  for  a  distance  of  chord 
equal  to  that  used  in  the  drawings  of  the  sections.  This  enables  a 
rather  more  graphic  conception  to  be  obtained  than  has  been  possible 
heretofore.  The  values  of  KL  and  L/D  are  given  in  groups  of  four 
sections.  The  graphs  look  complicated,  but  they  are  merely  con- 
venient methods  of  tabulating  the  results,  and  the  curves  can  readily 
be  distinguished  with  a  little  practice  in  reading  off  the  values. 

Among  the  'sections,  given  the  Eiffel  No.  13  bis,  the  one  used  on 
the  Bleriot  monoplanes,  is  a  very  widely  adopted  one,  and  because 
of  its  high  lift  and  good  efficiency  it  is  one  of  the  few  of  the  older  types 
of  sections  remaining  in  use.  Many  of  the  Royal  Aircraft  Factory 
biplanes,  the  Bristol  biplane,  several  German  and  Italian  aeroplanes, 
and  the  Martin  biplane  in  this  country,  use  a  section  of  this  type,  t  Its 
most  serious  disadvantage  is  the  lack  of  spar  room,  necessitating  either 
a  wide  shallow  and,  therefore,  heavy  spar,  or  a  lesser  factor  of  safety 
on  a  well  loaded  wing.  The  efficiency  at  very  low  angles  is  not  as  good 
as  in  some  of  the  newer  types  of  sections,  which  permit  of  a  greater 
range  of  speed  though  not  possessing  quite  as  good  a  maximum  effi- 
ciency. 

The  Eiffel  No.  31  section,  of  crescent  shape,  is  Eiffel's  most  ef- 
ficient all-around  wing,  although  its  maximum  L/D  is  exceeded  by 
many  other  sections.  The  Lift  at  low  angles  is  very  high,  and  the  wing 
is  well  adapted  for  load-carrying  aeroplanes. 

No.  32  Eiffel  is  essentially  a  speed  range  wing,  for  fast  speed  scouts, 
lightly  loaded  and  with  high-powered  engines.  The  high  value  of 
L/D  at  low  angles  is  particularly  favorable  to  high  speed. 

No.  36,  Eiffel  is  used  on  several  military  machines,  and  is  a  partic- 
ularly good  wing  for  a  meduim  speed,  military  scout.  The  Lift  is 
not  run  up  very  high,  but  the  range  of  angles  thru  which  a  high  L/D 
is  maintained  is  favorable,  not  only  to  high  speed,  but  also  to  climb, 
as  will  later  be  explained,  when  consideration  is  given  to  the  complete 
aeroplane  as  a  unit. 

The  Dorand  wing,  Eiffel  No.  35,  is  similar  to  the  Wright  wing, 
and  gives  a  very  high  lift,  with  a  high  L/D  at  angles  from  3°  to  6°. 
The  small  thickness  of  the  section,  however,  does  not  make  this  wing 
very  favorable  from  the  standpoint  of  construction.  In  general,  thinner 
wings  are  the  more  efficient,  but  spar  room  is  a  very  necessary  element, 
and  efficiency  and  strength  must  be  compromised. 


82 


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85 

The  Howard  Wright  wing,  in  which  the  contour  is  stepped,  has 
been  used  on  the  White  seaplanes,  but  its  characteristics  are  not  very 
advantageous,  excepting  in  that  the  c.  p.  movement  is  practically 
stationary. 

The  Nieuport  and  Deperdussin  are  two  standard  wings,  the  lat- 
ter designed  particularly  for  racing  aeroplanes. 

R.  A.  F.  6  is  one  of  the  more  modern  sections  that  has  become 
standard  on  British  Army  aeroplanes,  and  also  used  on  the  huge  fly- 
ing-boat "America."  The  effect  of  a  reversal  of  the  trailing  edge  on 
this  section  is  shown  also,  and  is  of  interest  in  connection  with  flaps 
on  the  trailing  edge. 

The  N.  P.  L.,  No.  4  wing  is  a  particularly  deep  one,  in  which  the 
high  Lift  and  fairly  good  L/D  at  angles  of  3°  to  6°,  are  advantageous 
for  aeroplanes  having  a  slow  mean  speed. 

A  new  type  of  section,  with  a  movable  rear  piece,  is  also  shown, 
as  a  suggestion  of  improvement  by  the  writer.  The  combination  of 
low  Lift  and  good  L/D  of  a  flatter  section,  at  low  angle,  with  facilities 
for  changing  to  a  deeply  cambered  surface,  which  would  have  a  high 
Lift  and  also  a  high  L/D  at  larger  angles,  could  be  made  very  greatly 
to  extend  the  speed  range  of  aeroplanes.  Suggested  curves  of  a  pre- 
diction of  the  characteristics  of  a  surface  of  this  kind  are  indicated. 
It  should  be  emphasized  here,  that  several  years  ago  the  extent  to  which 
Lift  and  L/D  could  be  varied  on  sections  was  not  well  known,  and 
many  investigators  looked  for  an  extension  of  speed  range,  by  varying 
the  size  of  the  surface.  The  latest  experiments  indicate,  however, 
that  since  a  change  in  section  can  be  made  to  vary  the  Lift  and  L/D, 
100  per  cent,  or  more,  at  different  angles,  much  more  is  to  be  expected 
from  a  variable  curvature  section  in  extending  speed  range. 

The  Tail  Planes. 

The  main  wing  surfaces  determine  in  large  measure  the  general 
characteristics  of  an  aeroplane,  but  the  Lift  and  Drift  of  the  tail  pieces 
or  "empennages"  are  by  no  means  negligible.  The  characteristic 
variations  and  values  of  L  and  L/D,  that  have  been  given,  are  sufficiently 
complete  to  enable  us  to  determine  their  magnitude  for  these  aux- 
iliary surfaces,  when  it  is  realized  that  the  effect  of  the  propeller  stream, 
on  the  empennages  is  a  powerful  but  more  or  less  indeterminate  factor. 

Where  balanced  rudders  are  used,  consisting  of  a  flat  surface  of 
a  certain  aspect  ratio,  it  is  merely  necessary  to  apply  the  data  given 
on  p.  60.  And,  as  is  often  the  case,  where  a  pivoted  balanced  rudder 
is  of  a  more  streamlined  section,  as  illustrated  on  p.  78,  it  is  proper 
to  consider  the  drift  slightly  reduced.  Elevators  or  ailerons,  con- 
sisting of  a  balanced  surface  of  constant  chord,  span  and  section,  pivoted 


to  take  various  angles  of  incidence,  may  be  solved  by  the  data  given 
for  their  particular  section. 

On  some  aeroplanes,  notably  the  early  Wright  biplanes,  the  ele- 
vator consisted  of  a  normally  flat  plane  that  was  quite  flexible.  This 
surface  was  fixed,  at  the  leading  edge,  and  so  connected  at  the  trail- 
ing edge  that  movement  for  control  consisted  of  bending  the  ribs  by 
moving  the  trailing  edge  up  or  down,  thus  causing  the  section  to  take 
various  curvatures  and  angles.  A  surface  of  this  kind  is  readily  solved 
by  applying  the  data  given  on  p.  66,  for  sections  of  varying  camber. 

The  more  usual  type  of  elevator,  however,  is  the  "flap  and  fin" 
type,  in  which  movable  flaps  are  hinged  to  the  rear  .of  a  fixed  surface. 
It  has  often  been  customary  to  consider  these  surfaces  separately, 
but  a  moment's  thought  on  the  continuity  of  the  air  flow,  shows  that 
the  proper  conception  is  to  consider  a  surface  of  this  kind  altogether 
as  a  single  unit,  which,  when  the  flaps  are  in  line  with  the  fixed  por- 
tion gives  a  flat  surface  of  a  certain  aspect  ratio.  When  the  flaps  are 
moved,  there  is  obtained  a  section  that  is  arched  (though  not  circular), 
and  in  which  the  chord  is  a  line  from  the  trailing  edge  of  the  flaps  to 
the  leading  edge  of  the  fixed  plane,  with  a  camber  depending  on  the 
amount  the  flap  is  turned.  The  data  on  curved  sections  given  on  p.  64 
and  66,  is  then  applicable,  with  the  modification  that  the  section  being 
a  pointed  arch,  instead  of  circular,  will  have  a  somewhat  greater  Drift, 
though  the  Lift  may  be  taken  as  about  the  same. 

INTERFERENCE  OF  AEROFOILS. 


A  study  of  the  flow  of  the  air  stream  about  an  aerofoil  gives  a 
clear  indication  that  the  streamlines  are  influenced  and  deflected  quite 
a  distance  away  from  the  surface,  the  rising  streamline  caused  by  the 
"dipping"  front  edge  of  an  aerofoil  being  an  example.  In  addition, 
the  flow  causes  differences  in  pressure  on  an  aerofoil,  which,  if  affected, 
would  modify  the  total  forces  on  the  aerofoil. 

It  follows  that  placing  bodies  or  other  aerofoils  in  proximity  to 
any  aerofoil  will  greatly  affect  its  pressures.  Interferences  in  flow 
are  very  interesting,  and  of  most  practical  value,  in  their  application 
to  the  aeroplane. 

Biplane  Effect. 

When  aerofoils  are  placed  over  one  another,  as  in  a  biplane,  there 
results  an  interference  and  modification  of  their  air  forces.     It  is  cus-1 
ternary  to  refer  to  the  distance  apart  of  the  two  superposed  surfaces, 
as  the  gap,  and  the  ratio  of  gap  to  chord,  is  used  as  a  measure  thereof. 

Since  the  suction  on  the  upper  face,  is  about  three  times  as  great 
as  the  compression  on  the  lower  face,  of  an  aerofoil,  the  effect  of  plac- 
ing one  over  the  other  is  greatly  to  reduce  the  Lift  and  efficiency  of 

'• 


87 


the  lower  plane,  but  only  slightly  to  affect  the  upper  plane.  This 
is  evident  when  it  is  borne  in  mind  that  the  compression  on  the  bot- 
tom face  of  the  upper  aerofoil  and  the  suction  on  the  top  face  of  the 
lower  aerofoil  merge  into  and  mutually  reduce  each  other,  whereas 
the  suction  on  the  top  face  of  the  upper  aerofoil  and  compression  on 
the  bottom  of  the  lower  aerofoil  remain  unaltered.  The  suction  be- 
ing so  much  more  important,  it  follows  that  the  upper  aerofoil  must 
be  much  less  affected.  This  is  verified  by  the  laboratories,  and  prac- 
tically the  entire  loss  due  to  biplane  effect  is  found  in  reduction  of  L 
and  L/D  of  the  lower  surface.  A  deduction  to  be  drawn  from  this 
is,  that  flaps  on  the  upper  plane  are  much  more  effective  than  flaps 
on  the  lower.  Also,  flatter  planes,,  in  which  the  suction  is  not  so  great, 
would  be  less  interfered  with  when  superposed.  If  the  combination 
of  high  camber  upper  plane  and  a  very  much  flatter  lower  plane,  were 
used,  it  is  evident  that  the  interference  would  be  reduced  consider- 
ably. A  table  of  biplane  reduction  coefficients  for  an  average  aerofoil 
is  given. 

N.  P.  L.  BIPLANE  TABLE. 
To  obtain  values  for  a  biplane,  multiply  values  for  single  aerofoil  by  factors  given. 


BIPLANE 

LIFT 

LIFT/DRIFT 

SPACING 

GAP   CHORD 

6° 

8° 

10° 

6° 

8° 

10° 

0.4 

.61 

.63 

.62 

.75 

.81 

.84 

0.8 

.76           .78 

.77 

.79 

.82 

.86 

10 

.81             .82 

.82 

.81 

.84 

.87 

1.2 

.86            .87 

.86 

.84 

.85 

.88 

1.6 

.89 

.90 

.89 

.88 

.89 

.91 

Staggering. 

The  position  of  biplane  surfaces  over  each  other  is  subject  to  vari- 
ation, and  the  term  stagger  is  used  to  describe  the  relative  position 
referred  to  the  vertical.  For  reasons  of  visibility,  and  minor  consid- 
erations of  construction  and  balance,  it  is  sometimes  convenient  to 
stagger  the  upper  plane  ahead  of  the  lower  plane,  as  indicated  in  the 
sketch  on  p.  78.  The  effect  of  staggering,  on  the  efficiency  of  the  aero- 
foils, is  again  an  illustration  of  the  mutual  reaction  of  the  regions 
of  suction  and  compression.  When  the  upper  plane  is  staggered  for- 
ward, its  Lift  and  L/D  are  improved,  but  at  the  same  time  the  L/D 
on  the  lower  plane  is  reduced.  When  the  stagger  is  .44  of  the  chord 


88 

(a  practical  limit),  the  total  effect  is  to  cause  the  Lift,  on  the  biplane 
as  a  unit  at  angles  of  5°  to  10°,  to  be  improved  by  about  7%  to  9% 
with  practically  no  effect  on  the  L/D. 

Interference  of  Following  Planes. 

The  air  stream  deflected  from  the  main  aerofoils  of  an  aeroplane, 
takes  a  downward  course,  which  causes  the  air  flow  past  the  empen- 
nages, or  any  surfaces  in  the  rear,  to  be  affected,  and  causing  the  angles 
of  incidence  of  the  rear  surfaces  (which  are  always  the  angles  of  the 
chord  with  the  air  stream)  to  be  less  than  the  angles  of  their  chords 
with  the  horizontal  flight  axis.  This  is  an  exceedingly  important  ele- 
ment in  the  balance  and  stability  of  a  machine,  and  is  taken  up,  more 
fully,  in  considering  the  entire  aeroplane  as  a  unit  further  on. 

Dihedral  and  Retreat. 

Attention  is  called  to  the  definitions  of  Dihedral  angle  and  Re- 
treat, given  graphically  on  p.  78.  The  effect  of  these  features  is  con- 
sidered later  with  reference  to  stability.  Within  the  limits  used  in 
practice  their  effect  on  Lift  and  Drift  is  negligible. 

Summary 

From  a  combined  consideration  of  Aspect  Ratio,  Biplane  effect 
and  staggering,  a  biplane  at  6°  of  aspect  6,  stagger  of  .44  chord  and 
gap  equal  to  chord,  would  have  about  89%  of  the  lift  of  a  single  aero- 
foil (81%  due  to  biplane  effect  and  8%  increase  due  to  stagger)  and 
its  L/D  would  be  81%  of  that  of  a  single  aeroplane  of  the  same  as- 
pect ratio.  If  this  is  compared  with  a  single  aerofoil  of  aspect  4.5, 
however,  it  is  found  that  the  Lift  is  practically  the  same,  and  only 
a  slight  difference  is  found  in  the  efficiency.  Likewise,  a  staggered 
biplane  of  aspect  8  and  a  large  gap,  is  practically  the  same  as  a  mono- 
plane of  aspect  6. 

When  a  comparison,  like  the  above,  is  made,  the  reference  to  single 
aerofoil  means  an  equivalent  monoplane  of  the  same  surface  area  as 
the  biplane.  To  get  the  same  lift  with  the  same  section  and  aspect, 
a  monoplane  would  require  less  area  than  a  biplane,  by  the  amount  of 
the  biplane  coefficient. 

The  data  given  on  surfaces  enables  the  lifting  capacity  and  cor- 
responding wing  resistance  to  be  determined  for  the  various  sections. 
Examples  indicating  the  manner  in  which  this  data  is  used,  and  a  con- 
sideration of  the  aeroplane  as  a  unit,  may  now  be  taken  up. 


CHAPTER   VIII. 
CHARACTERISTICS    OF   THE   AEROPLANE. 

The  surprising  accuracy  with  which  the  performances  of  an  aero- 
plane may  be  predicted  from  data  on  the  lifts  and  resistances  of  its 
component  parts,  is,  perhaps,  the  most  striking  indication  of  the  great 
progress  that  has  been  made  in  Aeronautical  Engineering,  the  past 
year  or  two.  Constructors,  fliers  and  the  laboratories,  have  co-op- 
erated to  advantage,  and  although  many  important  features  of  the 
aeroplane  remain  to  be  explored,  information  that  already  has  been 
obtained  and  verified,  by  the  great  work  of  the  Laboratories,  readily 
permits  of  establishing  a  working  basis  for  the  presentation  of  data 
of  importance,  relative  to  the  aeroplane,  —  in  a  manner  not  only  use- 
ful and  intelligible  to  the  aeroplane  user,  but  at  the  same  time  capable 
of  expansion  as  new  conceptions  develop. 

It  is  proposed  in  this  chapter  to  consider  the  aeroplane  as  a  unit, 
with  a  view  to  determination  of  its  total  lifting  capacity  and  resist- 
ances and  the  power  necessary  to  fly.  In  a  treatise  on  aeroplane  de- 
sign, the  matter  considered  here  in  a  few  pages  would  of  itself  consti- 
tute a  text  book,  so  that  the  limiting  scope  of  this  work  makes  it  neces- 
sary to  confine  our  attention  to  the  military  "field  use"  features  capable 
of  leading  to  an  intelligent  solution  of  problems  in  the  modification 
of  aeroplanes  and  their  performances,  as  dictated  by  military  neces- 
sity. Flying  various  types  of  machines,  with  greatly  varying  load 
conditions,  radius  of  action,  atmospheric  conditions,  and  power  varia- 
tions, presents  a  vast  quantity  of  problems  that  often  are  solved  best 
by  the  fliers  themselves.  That  new  kind  of  resourcefulness,  in  adapt- 
ing themselves  to  many  changing  requirements,  that  is  demanded 
of  a  Flying  Corps,  is  a  criterion  of  efficiency  and  may  be  gauged  not 
only  by  skill  in  maintenance,  but  also  by  the  knowledge  that  the  avia- 
tors and  mechanicians  have  of  the  performances  that  may  be  expected 
of  their  machines. 

It  must  be  borne  in  mind  that  a  manufacturer  is  required  to  fur- 
nish data  on  his  machine  in  detail,  and  although  a  few  examples  are 
given  here,  information  on  the  resistances,  lifts,  power  available,  and 
power  required  to  fly,  under  definite  conditions,  of  particular  types, 
should  come  with  each  machine  —  the  manufacturer  in  other  words, 
interpreting  the  laboratory  results  applied  to  his  type,  for  the  benefit 
of  the  user.  It  is  clear,  therefore,  that  the  military  or  naval  user  of  an 
aeroplane  must  know  how  to  read  this  data  and  how  to  apply  it  in  a 
practical  way. 


90 

In  previous  chapters,  consideration  has  been  given  to  the  resist- 
ances of  bodies,  and  the  lifting  efficiency  of  surfaces  and  aerofoils  — 
completely  enough,  to  explain  the  significance  of  the  forces  generated 
by  an  air  stream,  and  with  sufficient  laboratory  data  to  make  the  sub- 
ject matter  of  direct  value  for  reference.  We  are  now  at  liberty  to 
combine  these  conceptions,  and  to  give  the  definition  of  an  aeroplane, 
(p.  11)  a  more  technical  wording  —  in  that,  an  aeroplane  consists  of  a 
combination  of  sustaining  and  balancing  aerofoils,  with  a  Lift  deter- 
mined by  the  values  of  KL,V  and  S,  and  with  power  suitably  proportioned 
to  overcome  the  head  resistance  of  the  structure,  and  the  Drift  of  the 
wings,  at  the  expense  of  which  the  Lift  is  obtained. 

Types   of   Aeroplanes. 

Reference  to  Chap.  II,  gives  a  renewed  significance  to  the  photo- 
graphs of  the  various  types  of  aeroplanes,  and  could  profitably  be  re- 
considered with  a  view 'to  fixing  the  relation  of  theory  and  practice. 
Thus,  the  wing  section  of  the  Curtiss  Tractor,  on  p.  17,  is  none  other 
than  Aerofoil  No.  36,  of  Eiffel,  given  on  p.  82,  and  the  wings  of  the 
monocoque  on  p.  20,  have  a  section  identical  with  Aerofoil  No.  54, 
defined  on  p.  83.  The  several  machines  differ  widely  in  values  of  the 
resistances  of  their  various  structural  parts.  Thus,  the  struts  on  the 
old-type  Wright  Aeroplanes,  shown  on  p.  19,  have  something  like  five 
times  the  resistance  of  the  struts  on  the  Sturtevant  tractor,  p.  24,  and 
the  wheels  on  the  Curtiss  Tractor,  p.  17,  may  be  expected  to  have 
about  half  the  resistance  of  the  wheels  on  the  Signal  Corps  tractor, 
shown  below  it,  due  to  covering.  The  maze  of  wires  and  struts  on 
the  old  types  of  pusher  biplanes,  are  obviously  more  resisting  than 
the  simplified  bracing  and  covered  bodies  of  the  later  types.  The  dif- 
ference in  aspect  ratio  of  the  Bleriot,  on  p.  20,  and  the  upper  plane  of 
the  Farman,  on  p.  19,  is  most  noticeable.  And,  whereas,  the  Curtiss 
Model  N  has  two  staggered  planes  and  a  dihedral  angle,  the  Deper- 
dussin,  on  p.  20,  has  a  single  surface  with  no  dihedral.  And  yet  if  the 
surface  section  were  the  same,  as  is  the  case  with  the  Bleriot,  p.  20, 
and  the  Martin,  p.  15,  we  would  apply  the  same  aerofoil  data  to  both  of 
them,  with  suitable  corrections  for  Aspect  Ratio,  biplane  interference 
and  stagger.  In  addition,  it  may  be  noticed  that  the  shapes  of  the  fusel- 
ages, differ  considerably,  some  tapering  to  an  edge  horizontally  and 
others  vertically,  some  square,  others  round,  etc. 

Each  aeroplane,  therefore,  is  bound  to  have  particular  character- 
istics of  its  own,  for  each  of  which  the  designer,  if  competent,  had  some 
particular  object  in  view,  towards  either  efficiency,  stability,  strength 
or  convenience.  To  investigate  them  all  would  be  a  trespass  on  the 
domain  of  the  aeronautical  engineer.  But  not  to  appreciate  what 
performances  may  be  expected  of  any  machine,  is  due  to  a  lack  of  infor- 
mation, that  it  is  the  object  of  this  work  to  supply. 


91 


From  the  standpoint  of  lifts,  resistances  and  power  required,  the 
many  different  types  all  resemble  each  other  in  having  a  set  of  main 
supporting  surfaces,  auxiliary  balancing  surfaces,  which  may  or  may 
not  exert  lifting  pressure,  and  certain  structural  resistances.  In  power 
available,  there  are  differences  of  importance  due  to  gearing  of  the 
propellers.  Whereas,  in  characteristics  of  stability  and  operation, 
distinctions  are  most  pronounced,  and  necessitate  a  full  consideration 
later. 

But  whether  tractors,  pushers,  staggered  biplanes,  monoplane 
aeroboats,  etc.,  all  aeroplanes  have  these  characteristics  in  common: 

I.  A  Lifting  Capacity,  determined  by  the  surface  characteristics, 

and  varying  with  speed  and  inclination  of  the  machine. 

II.  A  total  resistance  to  motion,  composed  of 

(a)  The    combined    resistances    of    the    various    necessary 
structural    parts,     called    the     Structural    Resistance, 
and  varying  with  speed  and  inclination. 

(b)  The  Drift,  which  is  determined  solely  by  the  Lift  char- 
acteristics and  is,  of  itself,  independent  of  speed. 

III.  A  certain  Power  Required  to  fly,  varying  with  the  speeds 

of  the  machine  and  its  total  resistances. 

IV.  A  certain  Power  Available,  due  entirely  to  the  horse-power 

given  out  by  the  propeller,  which,  in  turn,  for  various  speeds 
is  a  certain  proportion  of  the  power  of  the  engine,  and  there- 
fore must  correspond  to  a  certain  fuel  consumption. 
Flight  is  impossible  unless  the  Lifting  capacity  exceeds  the  total 
weight,  and  the  Power  Available  is  greater  than  the  Power  Required. 
A  study  of  these  features  enables  the  speed  range,  the  glide,  the 
climbing  rate,  the  load-lifting  capacity  and  the  fuel  consumption,  to 
be  determined  in  a  most  practical  manner. 


A  STURTEVANT  AEROPLANE    RISING  OFF   THE   GROUND 


92 
Inclination   of  the   Aeroplane. 

The  variations  of  the  pressures  on  surfaces  has  been  considered 
for  changes  in  the  angle  of  incidence.  It  is  customary  in  aeroplanes 
likewise  to  refer  to  "angle  of  incidence,"  of  the  supporting  surfaces, 
in  defining  the  attitude  of  the  .machine.  And  the  inclination  of  the 
body  to  the  line  of  flight,  and  the  line  of  the  propeller  axis,  is  referred 
to  as  the  angle  of  incidence.  If  the  wing  is  set  at  5°  to  the  axis  of  the 
body,  and  the  angle  of  incidence  of  the  machine  is  5°,  it  follows  that 
the  body  lies  parallel  to  the  air-flow.  Whereas,  if  this  same  machine 
were  presented  to  the  air  at  10°  incidence,  the  body  axis  would  make 
an  angle  of  5°  with  the  air-flow.  It  is  of  importance,  now,  to  realize 
that  the  entire  aeroplane  as  a  unit  may  be  presented  to  the  air  at  various 
inclinations. 

On  p.  13  the  three  motions  an  aeroplane  is  subject  to  —  pitching, 
yawing  and  rolling  —  are  defined.  On  practically  all  aeroplanes  the 
lifting  planes  are  fixed  to  the  body,  so  that  a  variation  in  angle  of  in- 
cidence means  pitching  of  the  machine  and  is  considered  more  fully 
here  than  either  yawing  or  rolling,  because  of  the  effect  change  of  in- 
cidence has  on  the  surface  characteristics.  Yawing  slightly  affects 
the  resistances,  and  rolling  may  affect  the  Lift,  but  both  are  more  pro- 
perly considered  under  Stability. 

The  auxiliary  surfaces,  particularly  the  tail-planes,  are  in  turn 
affected  by  the  pitching  of  the  machine,  or,  as  we  have  defined  it,  by 
changes  in  the  angle  of  incidence  of  the  aeroplane.  Where  the  ma- 
chine is  so  balanced  that  the  tail  lifts,  then  as  the  incidence  of  the  aero- 
plane is  increased  the  lift  of  the  tail  surfaces  increases.  And  if  the 
tail  is  set  to  receive  a  downward  pressure,  an  increase  of  incidence  causes 
this  to  be  relieved. 

As  will  be  seen  later,  the  variation  in  inclination  of  the  structure, 
at  the  different  angles  of  incidence,  gives  rise  to  alterations  in  the  struc- 
tural air  resistance,  particularly  of  the  fuselage,  and  in  a  staggered 
biplane  an  increase  in  the  angle  of  incidence,  increases  the  resistance 
of  the  struts  and  wires. 

The  Aeroplane  as  a  combination,  then,  must  be  studied  at  various 
attitudes,  and  changes  of  inclination  are  expressed  as  changes  in  angle 
of  incidence  of  the  supporting  planes.  Where  the  special  feature  is 
involved  of  varying  the  angle  of  incidence  as  on  some  recent  machines, 
inclination  could  be  referred  to  the  propeller  axis.  But  it  is  more 
convenient  in  determining  Resistances,  and  Lifts,  to  consider  the  chord 
of  the  wing  as  the  base  line. 

Before  proceeding  with  the  study  of  Resistance  and  Power  charac- 
teristics of  an  aeroplane,  attention  must  be  given  to  important  features 
occasioned  by  combinations  of  lifting  and  auxiliary  aerofoils,  on  the 
aeroplane  frame. 


93 
Decalage,  Wash-out,  and  Tail  Interference. 

The  term  "decalage"  is  used  to  define  the  difference  in  the  angle 
of  incidence  between  any  two  distinct  aerofoils  on  an  aeroplane.  It 
is  most  often  used  to  describe  the  difference  between  the  setting  of 
the  main  planes  and  the  tail  piece,  and  in  a  biplane  the  term  is  also 
used  to  denote  a  difference  in  angle  of  incidence  between  the  upper 
surface  and  the  lower  one.  Thus,  on  an  aeroplane  in  which  the  body 
axis  is  in  the  line  of  flight  with  an  angle  of  incidence  of  5°,  and  with 
the  chord  of  the  elevator,  inclined  +2°  above  the  body  axis,  the  deca- 
lage of  the  elevator  would  be  3°.  And  in  a  biplane  where,  in  order  to 
gain  slightly  in  efficiency,  the  upper  surface  is  set  at  an  incidence  of 
3°,  when  the  lower  one  is  at  5°,  the  decalage  would  be  equal  to  2°. 

With  reference  to  the  decalage  of  the  surfaces  of  a  staggered  bi- 
plane, laboratory  experiments  indicate  that  the  effect  of  setting  the 
upper  surface  at  about  2°  less  incidence  than  the  lower  surface  gives 
a  pronounced  increase  in  Lift  and  a  slight  gain  in  L/D  over  any  other 
setting.  This,  however,  is  subject  to  modification  where  different 
wing  sections  are  used,  and  a  field  of  importance  remains  to  be  explored 
in  the  determination  of  the  best  combination  of  stagger,  surface  sec- 
tions and  decalage,  to  minimize  the  effect  of  biplane  interference  and 
improve  the  Lift  range  of  the  biplane  as  a  unit. 

"Wash-out"  is  a  term  used  to  describe  the  progressive  reduction 
in  the  angle  of  incidence,  from  body  to  tip,  used  on  some  aeroplanes 
for  reasons  of  stability.  Thus,  on  an  aeroplane,  in  which  the  wings 
are  set  at  an  angle  of  incidence  of  7°  at  the  body,  and  then  steadily 
reduced  until  the  angle  at  the  tip  is  only  3°,  there  is  said  to  be  a  "wash- 
out" of  4°.  With  reference  to  the  aerodynamic  characteristics  of  this 
feature,  laboratory  results  show  that  the  approximation  of  considering 
the  entire  wing,  as  set  at  an  incidence,  equal  to  the  mean  of  the  angles 
at  the  body  and  the  tip,  is  quite  close  enough.  The  stability  features 
will  be  given  consideration  later. 

The  air  that  is  thrown  back  from  the  front  main  surfaces  of  an 
aeroplane,  onto  the  tail,  is  given  a  most  pronounced  downward  trend, 
governed  by  the  particular  angle  of  incidence  and  surface  section  com- 
bination used.  '  The  tail  pieces,  consequently,  are  riding  in  air  waves 
generated  by  the  sustaining  surfaces,  and  therefore  are  interfered  with. 
"Tail  interference"  has  only  recently  been  given  proper  considera- 
tion, and  its  importance  on  the  functioning  of  a  machine  requires  parti- 
cular attention.  Eiffel's  experiments  on  this  feature  are  particularly 
complete,  and  from  them  there  can  be  drawn  the  general  conclusion 
that  the  air,  passing  back  from  the  sustaining  surfaces,  acquires  a  down- 
ward trend,  dependent  on  their  angle  of  incidence,  which  persists  for 
some  time,  so  that  by  the  time  this  air  region  passes  by  the  tail  sur- 
faces it  has  straightened  out  to  only  a  half  to  one  degree  less  than  the 


94 

actual  angle  of  incidence  of  the  sustaining  planes.  It  becomes  nec- 
essary, then,  to  distinguish  between  the  apparent  angle  of  incidence 
of  the  tail  surfaces  and  their  real  angle  with  the  direction  of  the  air- 
flow past  them.  The  apparent  angle  is  the  incidence  referred  to  the 
line  of  flight,  just  as  for  the  sustaining  planes,  whereas  the  real 
angle  at  which  the  air  attacks  the  sustaining  planes  is  the  one  for  which 
all  calculations  of  pressures  on  the  tail  surfaces  must  be  made.  A 
few  examples  will  aid  in  making  this  clear.  Let  us  consider  an  aero- 
plane, at  an  angle  of  incidence  of  4°,  in  which  the  body  axis  is  parallel 
to  the  line  of  flight,  and  the  tail  surfaces  of  which  have  a  decalage  of  4°, 
with  the  sustaining  surfaces.  From  our  definition  of  decalage,  the 
apparent  angle  of  incidence  of  the  tail  surfaces  would  be  0°,  i.e.,  they 
lie  parallel  to  the  body  axis.  But  the  air  acquiring  a  downward  trend 
from  the  main  surfaces,  of  4°,  which  gradually  straightens  out  to  3°, 
as  it  passes  the  tail  causes  the  real  angle  of  attack  of  the  air  on  the  tail 
surfaces  to  be  —3°.  For  the  same  case,  if  the  tail  surfaces  are  acted  upon 
by  the  air  stream,  so  that  their  real  angle  of  incidence  is  0°,  it  follows 
that  the  sustaining  surfaces  are  at  an  angle  of  incidence  of  +7°  and  the 
body  is  inclined  to  the  air  flow  at  +3°.  For  any  particular  machine, 
it  is  necessary  to  have  special  data  on  these  features  furnished  by  the 
designer. 

Although  other  features  causing  modification  of  pressures  on  the 
various  aerofoils  may  be  met  with,  their  importance  would  not  re- 
quire special  consideration  here.  The  type  of  sustaining  surface  char- 
acterized by  the  "Dunne"  class  of  aeroplanes,  see  p.  23,  is  readily  solved 
when  the  laboratory  data  on  this  surface  as  a  unit  is  furnished,  —  since 
the  changing  camber,  and  angle  of  incidence,  would  in  no  way  alter 
the  method  of  considering  the  values  of  L  and  L/D  at  different  angles 
of  inclination,  precisely  as  for  any  other  surface  section.  The  sta- 
bility features  of  this  type,  however,  require  special  consideration. 

Having  acquired  a  working  conception  of  the  aeroplane  as  a  unit, 
we  may  proceed  with  a  study  of  its  probable  performances,  as  out- 
lined on  p.  91,  and  predicted  from  the  laboratory  measurements. 


I.    THE  LIFTING  CAPACITY. 

The  data  on  surface  sections  furnished  for  any  machine,  together 
with  suitable  corrections  for  biplane  effect,  aspect  ratio,  stagger,  deca- 
lage, etc.,  is  the  first  essential  —  and  perhaps  the  most  convenient 
way  to  represent  this  is  to  have  curves  showing  the  corrected  values 
of  L  and  L/D  as  applied  to  the  particular  machine,  on  the  same  chart 
with  the  data  on  the  wing  section  alone,  examples  of  which  are  given 
on  pp.  82-84.  The  corrected  curves,  then,  give  us  direct  informa- 
tion on  the  actual  values  of  KL  and  L/D,  to  apply  to  the  lifting  sur- 
faces as  a  unit,  corresponding  to  angles  of  incidence  of  the  chord  of 
the  wings  to  the  line  of  flight.  Since  the  value  of  the  surface  area 


95 

S  is  given  for  a  definite  machine,  and  also  information  on  the  weight 
W  to  be  carried,  we  can  at  once  supply  suitable  values  for  solving 

W  =  L  =  KL  S  V2 

so  that  we  may  learn  at  what  angles  the  machine  must  be  flown,  for 
given  speeds,  or,  conversely,  how  fast  and  how  slow  we  could  go,  with 
a  definite  range  of  angle  of  incidence.  Since  features  of  stability  de- 
termine a  safe  limit  of  angles,  the  latter  problem  is  the  one  most  often 
met  with. 

Thus,  for  an  aeroplane  with  335  sq.  ft.  of  surface  area,  in  the  form 
of  a  staggered  biplane,  of  aspect  7,  and  gap  equal  to  chord,  and  with 
a  wing  section  corresponding  to  Eiffel  No.  53,  the  values  of  KL  and 
L/D  corrected,  would  be  shown,  as  indicated  on  p.  96.  For  this  ex- 
ample, let  us  find  the  speed  range  corresponding  to  a  range  in  the  angle 
of  incidence  from  1°  to  12°. 

At  the  low  angle  1°,  we  find  by  referring  to  the  first  chart  that 
KL  =  .00085.  Supplying  values  of  S  and  L,  equal  to  the  weight,  we 
obtain 

W  =  KL  S  V2  =  1800  =  .00085  x  335  x  V2 
from  which  it  develops  that, 

V2  =  6320,  and  V  =  79.5  miles  per  hour. 

In  the  same  way,  reference  to  the  chart  of  aerofoil  characteristics 
shows  that  at  12°,  KL  =  .0027,  so  that 

L  =  1800  =  .0027  x  335  x  V2 
from  which  we  obtain, 

V  =  44.6  miles  per  hour. 

Since  the  required  lifting  power  and  area  of  the  wing  surfaces 
are  fixed,  it  is  hardly  necessary  to  emphasize  that,  for  any  inclination, 
there  is  only  one  speed  at  which  horizontal  flight  is  attained  with  the 
given  load.  Each  angle  'has  its  particular  corresponding  speed,  and 
in  the  above  example  the  angle  range  of  1°  to  12°,  corresponds  to  a 
speed  range  of  44.6  to  79.5  miles  per  hour,  and  to  none  other  —  unless 
the  load  is  changed  or  the  surface  characteristics  altered. 

A  simple  way  to  record  this  process  is  to  write  the  speeds  cor- 
responding to  the  various  angles  on  the  curve  for  KL,  as  has  been  done 
in  the  example  given. 

This,  then,  is  the  first  step  in  determining  the  aeroplane's  char- 
acteristics, i.  e. :  —  finding  the  speeds  required  in  order  to  lift  the  weight, 
at  various  angles  of  incidence. 


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98 
II.     TOTAL   RESISTANCE  TO  MOTION. 

As  already  indicated  several  times,  the  resistance  overcome  by 
the  propeller  consists  of  two  distinct  items:  Structural  Resistance  and 
Drift. 

Structural   Resistance. 

The  air  resistance  of  the  structural  parts  of  an  aeroplane,  such 
as  the  wheels,  struts,  wires,  body,  tanks,  etc.,  all  total  up  to  a  formid- 
able value,  and  are  conveniently  and  properly  classed  together  in  one 
item,  called  the  "Structural  Air  Resistance."  This  term,  altho  a  new 
one,  is  deemed  so  much  more  expressive  than  the  older  terms,  "body 
resistance,"  "parasite  resistance,"  etc.,  that  its  introduction  is  cer- 
tainly justified.  The  term  "parasite"  is  misleading,  since  a  high  drift 
is  as  much  a  "parasite"  as  an  uncovered  wheel. 

In  Chapter  IV  the  determinations  of  the  resistances  of  various 
shaped  bodies  were  given  consideration.  For  any  aeroplane  it  is  neces- 
sary to  know  the  details  of  construction  before  a  working  total  of  the 
structural  air  resistances  can  be  determined. 

There  are  hardly  any  two  types  of  aeroplanes  with  the  same  shape 
of  body,  so  that  this  item,  above  all  others,  can  be  considered  but  from 
data  given  by  the  manufacturer.  It  may  be  of  interest  to  note,  how- 
ever, that  the  values  of  K,  for  the  nacelle  of  the  Farman  (illustrated 
on  p.  19),  has  been  found  by  Eiffel  to  be  .0014,  and  K  for  the  Deper- 
dussin  monocoque  (p.  20),  is  .001.  It  has  also  been  determined  that 
in  yawing  and  pitching  the  flat-sided  fuselage  has  an  appreciably  greater 
resistance  than  a  rounded  one. 

The  resistance  of  the  tail  surfaces  ordinarily  should  include  some 
allowance  for  the  drift  of  the  tail,  as  determined  by  the  particular  shape 
and  incidence  used.  Altho  this  should,  perhaps,  be  considered  in 
company  with  the  wing  resistances,  its  value  is  small  for  a  well-bal- 
anced machine,  and  it  is  more  convenient  to  include  it  in  the  struc- 
tural resistance  until  it  assumes  a  greater  value. 

Altho  the  process  of  determining  the  Structural  Resistance  con- 
sists essentially  of  applying  information  on  the  values  of  K,  for  the 
various  structural  items,  in  the  formula,  P  =  K  S  V2,  and  adding 
up  the  result,  we  have  found  that  a  change  in  V  for  horizontal  flight 
involves  a  change  in  the  angle  of  incidence.  This,  in  turn,  means 
that  at  the  various  speeds  the  entire  aeroplane  assumes  a  "tail  high  — 
nose  down,"  or  "tail  low  —  nose  high"  attitude.  For  the  wheels,  wires, 
etc.,  these  incidence  variations  have  but  a  slight  effect,  but  the  bodies, 
fuselages  or  nacelles  are  formed  so  as  to  give  a  least  resistance  in  only 
one  position  —  when  the  axis  is  in  line  with  the  wind.  Any  depart- 
ure from  this  due  to  a  change  in  the  angle  of  incidence,  causes  an  in- 
crease in  their  resistance.  So  that  at  angles  both  above  and  below 


the  normal  angle  of  incidence,  the  resistance  of  the  body  is  higher, 
due  to  higher  values  of  KL. 

On  p.  96  a  typical  resistance  chart  is  given,  and  on  it  is  shown  a 
typical  curve  of  structural  resistance.  The  range  of  incidence  of  1° 
to  12°,  used  as  an  example  already,  is,  as  indicated,  accompanied  by 
a  rise  in  structural  resistance  from  60  Ibs.  at  12°,  to  195  Ibs.  at  1°,  since 
the  speeds  corresponding  to  these  angles  are  44.6  and  79.5  m.  p.  h. 


Drift. 

If  we  refer  to  the  first  chart  showing  KL  and  L/D,  and  recall  that 
for  any  value  of  the  angle  of  incidence  the  value  of  KL  was  read,  to 
determine  speed,  it  is  seen  that  we  can  also  read  at  the  same  time  the 
value  of  L/D  for  that  particular  KL.  Knowing  the  weight,  this  ratio 
at  once  gives  us  the  Drift,  since  for  horizontal  flight, 

Drift  =  Weight  +  L/D. 

It  becomes  clear,  now,  why  reasons  of  convenience  lead  to  plot- 
ting the  values  of  L/D  in  preference  to  the  values  of  KD,  to  supply 
in  D  =  KD  S  V2.  Drift  is  always  a  fraction  of  Lift  and,  therefore,  of 
the  weight,  but  is  in  no  other  way  concerned  with  the  speed,  V. 

Thus,  when  the  chart  is  referred  to,  to  obtain  the  value  of  KL  for 
1°,  the  value  of  L/D  =  13.6  could  be  read  at  the  same  time,  and  know- 
ing that  the  weight  is  1800  Ibs.  the  Drift  at  that  angle  is  immediately 
determined  as,  1800  -f-  13.6  =  132  Ibs.  In  the  same  way  the  Drift  at 
12°  is  found  to  be,  1800  -  8  =  225  Ibs.,  and  the  least  drift  at  about  4° 
is  1800  -=-  15  =  120  Ibs.  Since  these  determinations  are  made  in  com- 
pany with  the  determinations  of  the  air  speeds  corresponding  to  the 
various  angles  of  incidence,  we  at  once  have  obtained  the  values  of 
the  Drift  for  the  various  speeds  —  and,  consequently,  have  solved  for 
the  second  part  of  the  Total  Air  Resistance.  We  proceed,  then,  to 
plot  a  curve  showing  the  Wing  Resistance,  on  the  same  chart,  on  which 
we  have  already  plotted  Structural  Resistance,  for  different  speeds. 

The  Total  Air  Resistance  is  the  sum  of  these  two.  On  a  chart, 
curves  drawn  to  the  same  cross  lines  are  easily  added  graphically,  by 
merely  surmounting  one  value  on  top  of  the  other.  Thus,  for  60  miles 
an  hour  speed,  the  value  of  the  Wing  Resistance,  120  Ibs.  is  added 
by  means  of  dividers  (in  actual  measurement)  above  the  point  on  the 
Structural  Resistance  curve,  which  reads  105  Ibs.,  and  this  gives  the 
total  225  Ibs.,  graphically.  The  same  process  is  followed  with  other 
points,  sufficient  to  establish  the  curve  of  Total  Resistance  to  motion, 
which  is  the  second  characteristic  to  be  determined. 


100 

III.     POWER  REQUIRED. 

In  Chapter  III  it  was  recalled  that  power  expended  corresponded 
to  the  exertion  of  foot  pounds  work  at  a  certain  rate,  and  that  one 
horse-power  =  550  foot  pounds  per  second. 

At  any  speed,  therefore,  the  Ibs.  Resistance  X  the  speed  in  feet 
pen  second,  gives  the  number  of  foot  pounds  per  second  used  up  by 
the*  aeroplane.  Dividing  this  quantity  by  550,  will  give  us  the  Horse 
Power  Required  for  horizontal  flight  at  that  particular  speed. 

If  we  call  R  the  resistance  and  V  the  speed  in  miles  per  hour,  then 

R  X  V  X  1.47 
H.P.  =  - 

550 

since  V  in  m.  p.  h.  must  be  multiplied  by  1.47,  in  order  to  express  it 
in  feet  per  second.  Combining  1.47  -r-  550,  we  get  the  handier  rela- 
tion that, 

Rx  V 
Required  H.  P.  =  - 

375 

where  R  is  the  total  Resistance  in  pounds  read  for  any  speed  from  the 
Resistance  Chart,  and  V  is  the  velocity  in  miles  per  hour. 

A  curve  may  then  be  plotted  of  Power  Required  to  fly  at  the  vari- 
ous speeds.  This  is  done  in  the  third  chart,  p.  96.  It  is  to  be  recalled 
that  in  plotting  the  Drift  on  the  resistance  chart,  the  corresponding 
angles  of  incidence  were  marked  on  the  curve. 

This  is  also  done  on  the  Power  Required  Curve,  the  correspond- 
ence between  Speeds  and  Angles  of  incidence  being  precisely  the  same 
as  originally  determined,  when  considering  the  first  chart  of  KL  and 
L/D. 

As  examples  of  the  manner  in  which  to  determine  Power  Required, 
let  us  take  the  machine  at  incidences  of  12°,  6°  and  1°. 

From  the  Resistance  Chart  we  find  that  at  12°,  corresponding 
to  a  speed  of  44 ^  m.  p.  h.,  the  Total  Resistance  is  285  Ibs.  Therefore, 

285  x  44.5 

Required  H.  P.  at  12°  =  -  -  =  33.8  h.  p. 

375 

The  same  values  read  for  6°,  give 

215  x  54 

Required  H.  P.  at  6°  =  -  -  =  31  h.  p. 

375 

And  likewise  for  1°,  there  is  obtained 

327  x  79.5 

Required  H.  P.  at  1°  =  -  -  =  69  h.  p. 

375 


101 

It  is  most  important  to  note  the  general  form  of  this  curve,  and 
as  a  "Characteristic"  of  the  aeroplane,  it  is  decidedly  the  most  im- 
portant one.  At  angles  below  10°  there  is  a  noticeable  rise  in  Power 
Required,  because  the  increase  in  Drift  is  so  much  greater  than  the 
decrease  in  Structural  Resistance,  corresponding  to  a  slower  speed. 
And  the  pronounced  increase  in  Power  Required,  at  angles  below  6°, 
is  due  primarily  to  the  greater  preponderance  of  the  increase  in  the 
Structural  Resistance,  as  the  speed  increases. 

At  angles  of  10°  to  6°,  corresponding  to  speeds  of  about  45  to  55 
m.  p.  h.,  the  Power  required  is  at  its  lowest  value  and  remains  very 
nearly  the  same  for  this  particular  machine.  Power  required  curves 
vary  greatly  for  different  aeroplanes,  both  in  their  contour  and  in  the 
angles  at  which  the  low  points  are  located.  But  the  rise  both  above 
and  below  a  certain  speed  where  the  power  is  least,  is  noticeable  on 
all  power  curves,  and  leads  to  the  general  conclusion,  that  high  drift 
at  low  speeds,  and  high  structural  resistance  at  high  speeds,  are  the 
wasteful  elements. 

The  establishment  of  all  the  points  on  the  Power  Required  Curve, 
is  made  in  the  manner  indicated,  and  we  then  obtain  the  third  char- 
acteristic of  the  Aeroplane  —  which  is  the  determination  of  the  horse 
power,  required  for  horizontal  flight,  at  various  speeds. 

IV.     POWER  AVAILABLE   FROM   THE  PROPELLER. 

A  certain  horse  power  is  given  by  the  engine  at  various  revolu- 
tions per  minute,  and  a  curve  of  this  "Brake  Horse  Power,"  for  cor- 
responding "r.  p.  m.,"  is  as  necessary  and  as  easily  furnished  as  in- 
formation on  the  size  and  weight  of  the  engine.  On  p.  97,  a  curve 
for  the  particular  motor  taken  as  an  example  here  is  given. 

But  this  power  is  not  directly  available,  since  its  exertion  on  the 
air  to  move  the  aeroplane  is  thru  the  medium  of  an  air  propeller,  which, 
unfortunately,  is  more  or  less  wasteful  of  the  power  the  engine  gives 
to  it. 

The  efficiency  of  the  propeller,  therefore,  must  be  considered. 
Of  all  features  of  the  aeroplane,  propeller  determinations  from  both 
theory  and  practice  are  exceedingly  unsatisfactory.  But  laboratory 
experiments,  notably  Eiffel's,  lately  have  given  valuable  informa- 
tion on  a  few  good  blades,  in  which  the  shape  and  section  are  left  un- 
altered, and  only  the  r.  p.  m.  and  diameter  adjusted  for  different  aero- 
planes. The  theory  of  the  "similitude  of  propellers,"  which  permits 
of  passing  from  one  machine  to  another  with  the  same  type  of  blade, 
is  at  present  the  only  really  valuable  basis  for  propeller  determina- 
tions. 

Experiment  shows  that  the  old  notion  of  "pitch,"  etc.,  on  a  basis 
of  screw  propeller  theory  is  poorly  founded.  Whereas,  the  more  mod- 


102 

ern  notion  of  a  propeller,  consists  simply  in  a  consideration  of  the  blade 
as  an  aerofoil  at  a  certain  angle  of  incidence,  moved  against  the  air 
in  a  rotating  path,  and  in  which  KL  S  V2,  would  represent  the  Thrust, 
and  KD  S  V2,  the  Torque. 

For  purposes  of  aeroplane  design,  considerations  of  the  propeller, 
its  loading,  deflections  and  strength,  and  its  Thrust  and  Torque  char- 
acteristics, are  most  important.  For  field  use  the  strength  question 
requires  merely  that  a  propeller  never  be  run  at  a  greater  r.  p.  m.  than 
has  been  proven  safe,  without  information  from  the  manufacturer  as 
to  the  strength  of  that  particular  propeller;  and  that  alterations,  such 
as  metal  tipping,  be  done  by  the  propeller  maker,  unless  the  propeller 
has  already  been  designed  therefor.  But,  we  are  very  vitally  inter- 
ested here  in  the  suitability  of  various  propellers,  for  different  aeroplane 
performances,  so  as  to  enable  us  to  pick  out  the  propeller  desired. 

Since  on  any  engine  the  power  is  determined  from  the  r.  p.  m.,  by 
merely  mounting  any  propeller  in  question  on  the  engine  and  reading 
the  r.  p.  m.  for  a  given  throttle,  there  is  at  once  established  the  power 
used  by  the  propeller.  This  is  so  readily  and  conveniently  done  in 
the  field  that  for  the  present  it  is  unnecessary  to  compute  by  extensive 
mathematics  the  power  necessary  to  drive  this  propeller  at  a  certain 
r.  p.  m.  In  the  determination  of  the  Power  given  out  by  the  propeller 
in  the  air,  however,  no  such  convenient  measurements  can  be  made. 
We  have  recourse,  therefore,  to  laboratory  data  furnished  with  the 
propeller. 

This  data  is  most  conveniently  given  as  -a  curve  showing  the  Effi- 
ciency of  the  propeller,  corresponding  to  values  of  the  quantity  v/nd. 

The  Efficiency  of  the  propeller  is  merely  the  %  of  the  power  put 
into  it,  that  is  given  out  in  Thrust  Power  by  the  propeller. 

The  quantity  v/nd,  is  a  convenient  numerical  relation,  used  by 
the  laboratories  to  express  the  Efficiency  of  a  propeller  of  definite  shape 
and  section  for  any  combination  of  values  of 

(1)  The  velocity  of  the  aeroplane  in  feet  per  second,  v, 

(2)  The  revolutions  per  second  of  the  engine,  n, 

(3)  The  diameter  of  the  propeller  in  feet,  d. 

The  speed  thru  the  air  of  the  tip  of  the  blade  is  determined  in 
feet  per  second  by  the  circumference  =  xd,  and  the  number  of  times 
a  second  it  covers  this  distance  =  n.  The  quantity  v/7rnd  is  the  actual 
relation  between  the  "tip  speed"  of  the  propeller  and  the  speed  thru 
the  air  of  the  entire  aeroplane.  Let  us  say,  briefly,  that  it  has  been 
"discovered"  that  this  relation  definitely  determines  the  efficiency  of 
any  particular  blade. 


103 

Our  data  on  the  engine  gives  us  n,  which  is  taken  in  this  example 
as  normally  1200  r.  p.  m.  =  20  r.  p.  s.  The  diameter,  d,  in  this  example 
is  8  feet.  For  any  speed  of  the  aeroplane,  v,  therefore,  we  can  com- 
pute v/nd,  and  on  the  Efficiency  chart,  p.  97,  read  %  efficiency  of 
the  propeller.  Knowing  the  horse  power  of  the  engine  for  the  given 
value  of  n,  we  readily  determine  the  actual  horse  power  available  from 
the  propeller.  As  an  example,  at  60  m.  p.  h.  speed,  v  =  60  x  1.47  = 
88  feet  per  second,  n  =  20,  and  d  =  8,  whence  v/nd  =  .55.  Reading  on 
the  first  chart  p.  97,  we  find  that  for  v/nd  =  .55,  propeller  efficiency  = 
76  %.  On  the  second  chart,  p.  97,  it  is  seen  that  at  20  revolutions 
per  second,  or  rather  20  x  60  =  1200  r.  p.  m.,  the  engine  may  be  ex- 
pected to  give  88  h.  p.  Our  propeller  Power  Available,  therefore,  is 
76  %  of  88  =  67  h.  p.,  and  is  so  plotted  on  the  Power  chart  for  1200 
r.  p.  m.,  on  p.  96. 

In  the  same  way  all  the  other  points,  not  only  for  this  same  curve 
but  for  values  of  r.  p.  m.  =  800,  1000,  etc.,  are  plotted,  and  we  thus 
obtain  the  fourth  characteristic  —  the  thrust  Power  Available  for  any 
r.  p.  m.,  at  the  various  speeds  of  the  aeroplane. 

PERFORMANCES  OF  THE  AEROPLANE. 

The  characteristics  of  the  aeroplane  having  been  determined  we 
may  proceed  with  determinations  of  the  performances  that  may  be 
expected  of  it. 

The    Glide   or   Volplane 

In  horizontal  flight  the  thrust  of  the  propeller  in  pounds  is  just 
slightly  in  excess  of  the  total  Resistance  of  the  Aeroplane.  When 
the  motor  is  shut  off,  however,  this  balance  between  power  required 
and  power  exerted  ceases,  and  a  distinctly  different  condition  of  flight 
results.  If  some  other  force  were  not  introduced  to  overcome  the 
total  resistance,  which  is  still  about  the  same  as  in  the  conditions  of 
power  flight,*  the  aeroplane  would  slow  down  and  finally  fall  in  some 
dangerously  unbalanced  condition.  Such  a  force  can  at  any  moment 
be  introduced,  by  merely  inclining  the  path  of  the  machine  down- 
wards, enough  to  cause  the  gravity  force,  equal  to  the  weight,  to  be- 
come the  resultant  of  two  forces  —  a  Lift  on  the  planes,  less  than  the 
weight  W,  and  a  forward  component  of  this  gravity  force,  equal  to  the 
Total  Resistance.  The  machine  then  descends,  on  a  downward  path, 
in  which  the  power  spent  in  descending  the  machine's  weight  at  an 
inclined  rate  corresponding  to  a  fall  of  a  certain  number  of  feet  per 
second  is  equal  to  the  power  used  up  in  overcoming  the  total  Resist- 
ance, at  the  particular  speed,  on  this  downward  path.  The  determi- 
nation of  the  slope  of  this  path,  becomes  very  easy.  It  is  merely  the 
ratio  of  the  Weight  of  the  machine  to  the  Total  Resistance,  at  the 
particular  angle  of  incidence  and  speed  assumed  on  the  glide.  This 

*  It  is  to  be  noted  that  in  a  tractor,  the  air  propeller  throws  back  a  stream  of  air 
on  the  body  that  has  a  speed  greater  than  the  aeroplane's  speed,  so  that  shutting  off  the 
engine  slightly  reduces  the  Total  Resistance. 


104 

feature  is  considered  again  in  connection  with  the  Stability  and  Opera- 
tion of  the  aeroplane. 

A  curve  of  "gliding  angles"  is  readily  plotted  on  the  Resistance 
Chart,  by  dividing  the  weight  by  the  Total  Resistance  at  any  point. 
Thus,  at  55  m.  p.  h.,  the  Total  Resistance  is  215  Ibs.  Therefore  the 
gliding  slope  is  8.4  to  1.  In  other  words,  the  aeroplane  will  travel 
8.4  times  as  far  as  its  vertical  descent. 

High  Speed  and  Low  Speed 

It  is  apparent  from  a  study  of  the  Power  Chart,  p.  96,  that  the 
speed  range  is  determined  by  the  crossing  points  of  the  Power  Re- 
quired and  Power  Available  curve.  Thus,  at  1200  r.  p.  m.,  horizontal 
flight  is  impossible  due  to  lack  of  power,  above  82  m.  p.  h.,  and  below 
41  m.  p.  h.  The  speed  ranges  for  other  r.  p.  m.  are  also  indicated. 

Climbing  Rate 

Although  atmospheric  conditions  vitally  affect  the  rate  of  climb 
and  height  attainable  of  any  aeroplane,  it  is  possible  to  determine  the 
initial  climbing  rate.  The  climbing  of  a  machine  is  due  to  the  exer- 
tion of  an  extra  amount  of  power,  which  raises  the  Ibs.  weight  of  the 
machine  a  certain  number  of  feet  per  second,  thereby  using  up  a  cer- 
tain horse  power.  This  excess  power  is  directly  available,  if  the  Power 
Available  is  greater  than  the  Power  Required.  And  a  measure  of  this 
excess  power  is  the  difference  between  these  two  curves.  Thus,  at  56 
m.  p.  h.,  the  Power  Required  is  32  h.  p.,  and  the  corresponding  Power 
Available  at  1200  r.  p.  m.  in  actual  thrust,  at  that  speed,  is  63  h.p. 
Therefore,  we  have  a  reserve  power  of  31  h.  p.,  which  can  be  entirely 
made  use  of  in  climbing  the  machine.  Since  the  weight  is  W  =  1800 
Ibs..  the  equation  for  climb  becomes, 

H.  P.  for  climb  =  1800  x  climbing  rate  in  feet  per  second,  whence, 
Climbing  Rate  =  H.  P.  in  foot  Ibs.  per  second  -4-  1800  Ibs.  weight. 
Therefore,  for  this  example, 

31  x  550 

Climb  in  feet  per  minute  =  —  —  x  60  =  570  f.  p.  m.,  rate. 

1800 

Summary 

Other  curves  giving  the  economy  in  fuel  consumption  and  cor- 
responding engine  speeds  and  aeroplane  speeds,  are  explained  on  p. 
97,  and  are  of  very  practical  value. 

By  the  processes  outlined  in  this  chapter,  the  performances  of 
an  aeroplane  may  be  predicted  and  recorded,  with  an  accuracy  and 
value  that  is,  indeed,  not  only  of  great  interest,  but  of  real  benefit  to 
the  aeroplane  user. 

It  is  seen  that  the  characteristics  of  the  aeroplane,  from  which 
the  performances  may  be  predicted  so  readily,  are  based  on  the  data 
furnished  by  the  laboratory  tests  on  the  aerodynamic  features  and 
the  engine,  so  that  the  significance  and  importance  of  this  informa- 
tion becomes  evident. 


CHAPTER   IX. 
STRESSES  AND   SAFETY  FACTORS. 


The  nature  and  magnitude  of  the  supporting  and  resisting  pres- 
sures on  aeroplanes,  and  their  effect  in  determining  characteristics 
and  performances  to  be  expected  when  the  thrust  power  available 
and  fuel  consumption  are  known,  constitute  one  feature  of  the  study 
of  the  aeroplane,  as  outlined  in  Chap.  IV.,  p.  41.  We  may  proceed, 
therefore,  with  a  consideration  of  the  second  feature  —  the  study  of 
the  construction  of  the  machine.  And  eventually,  after  having  given 
attention  to  stability  and  operation,  we  will  be  at  liberty  to  discuss 
the  various  military  types  of  aeroplanes. 

It  is  necessary  to  know  the  distributed  loading  on  the  aeroplane, 
of  the  air  forces  generated  by  the  movement  thru  the  air,  before  pro- 
per consideration  can  be  given  to  the  stresses  and  safety  factors  in  its 
structure. 

In  gliding,  the  lifting  forces  on  the  wings  are  slightly  less,  and 
in  climbing  slightly  greater,  than  in  horizontal  flight,  but  only  in  a 
small  degree.  When  attacked  by  sudden  puffs,  the  air  forces  are  in- 
creased in  various  ways;  banking  on  turns  introduces  extra  stresses, 
due  to  the  centripetal  force;  and  in  various  maneuvers  such  as  a  sud- 
den recovery  from  a  steep  dive,  looping  the  loop,  flying  with  full  power 
at  very  high  angles,  etc.,  additional  loads  are  imposed  on  the  structure 
of  the  machine,  which  must  be  withstood. 


Safety  Factor 

The  ratio  of  the  breaking  strength  of  any  structural  part  to  the 
load  imposed  upon  it,  is  termed  the  safety  factor  of  that  part.  Thus, 
if  a  wire  requires  a  tension  of  3000  Ibs.  in  order  to  break  it,  whereas 
the  load  it  carries  is  only  300  Ibs.,  it  is  said  to  have  a  safety  factor  of 
10.  In  ordinary  engineering  practice,  the  load  that  it  is  considered 
necessary  for  any  part  to  carry  is  taken  as  the  maximum  load  that 
the  particular  part  will  ever  have  to  stand,  and,  in  designing  it,  a  safety 
factor  is  applied  to  this  maximum  possible  load.  Contrary  to  all  good 
engineering  practice,  the  structural  parts  of  an  aeroplane  are  gener- 
ally designed  to  have  a  certain  "safety  factor,"  with  reference  to  the 
normal  flying  load,  determined  by  the  weight  of  the  machine.  The 


106 

excess  stress  due  to  some  additional  maneuver  is  taken  account  of 
in  the  "safety  factor"  itself,  so  that  in  the  engineering  sense  it  is  not 
a  safety  factor  at  all,  but  merely  an  allowance  for  extra  stresses,  in- 
duced by  conditions  other  than  ordinary  horizontal  flight.  It  is  pos- 
sible to  estimate  what  the  maximum  possible  stresses  are,  and  to  deter- 
mine whether  or  not  the  aeroplane  will  collapse  when  they  are  im- 
posed. And  in  general  an  aeroplane  is  so  designed  that  the  strength 
of  its  weakest  structural  part  will  at  least  be  great  enough  to  with- 
stand a  reasonable  value  of  this  maximum  stress,  without  breakage, 
the  real  safety  factor  being  very  seldom  as  much  as  two.  In  most 
other  branches  of  engineering  a  safety  factor  of  at  least  ten  is  required. 
The  object  of  a  safety  factor  is  to  provide  against  the  increased  stresses 
of  sudden  impact  shocks,  which  are  difficult  to  estimate,  and  to  take 
account  of  defective  material  and  workmanship,  so  that,  at  first  sight, 
it  would  seem  odd  that  intelligent  engineers  should  permit  this  gen- 
eral conception  of  "safety  factor"  in  aeroplanes  to  survive,  thereby 
apparently  still  further  increasing  the  dangers  of  aviation.  It  is  useless 
to  deny  this  element  of  danger,  or  to  attempt  to  excuse  it,  on  any  ground, 
excepting  that  it  is  a  well  considered  compromise  of  opposing  features. 


An  aeroplane,  constructed  with  a  high  safety  factor,  on  the  maxi- 
mum stresses  to  which  it  can  be  subjected,  would  actually  prove  so  poor 
and  dangerous  a  flyer  and  so  difficult  to  land,  due  to  its  enormous 
weight,  that  ever-present  dangers  and  limitations  in  its  operation 
would  far  outweigh  the  possible  dangers  of  its  not  being  quite  strong 
enough  to  stand  some  very  unusual  and  remote  maximum  stress,  to  which 
in  the  hands  of  a  well  informed  aviator  it  would  never  be  subjected. 
The  justification  for  building  aeroplanes  as  light  as  possible,  and  cut- 
ting down  to  the  limit  of  simplicity  and  necessity  all  the  structural 
features,  is  exactly  what  makes  a  well-built  aeroplane  one  of  the  most 
refined  of  engineering  structures.  The  fact  is  only  too  often  lost  sight 
of,  that  increasing  the  strength  of  an  aeroplane  for  flight,  by  thicker 
spars  and  struts,  heavier  wires,  cables  and  larger  fittings,  immediately 
requires  a  landing  gear  much  heavier  in  proportion,  all  of  which  results 
in  a  very  much  heavier  machine,  which  for  the  same  flying  character- 
istics will  require  a  more  powerful  engine,  not  only  heavier  in  itself, 
but  requiring  more  fuel,  larger  tanks,  etc.,  until  the  final  result  is  a 
machine  in  which  the  higher  safety  factor  is  largely  lost  by  greater 
stresses  due  to  the  increased  weight  —  with  nothing  gained.  In  aero- 
plane engineering  there  seems  to  be  a  remarkably  nice  balance  be- 
tween flying  capacity  and  limitations  of  strength  due  to  allowable 
weight  of  machine.  And  the  degree  in  which  strength  has  been  gained 
by  lightening  up  a  machine,  thereby  improving  its  flying  capacity,  is 
a  better  criterion  by  which  to  judge  of  an  aeroplane. 


107 
Maximum   Stresses. 

The  greatest  source  of  danger  in  flying,  due  to  imposing  great 
stress  on  the  wings,  is,  without  question,  given  rise  to  in  flattening 
out  sharply  after  a  long  dive.  Modern  aeroplanes  have  compara- 
tively low  structural  and  drift  resistance,  and  when  pointed  earth- 
wards the  gravity  force  of  the  weight  is  opposed  only  by  the  air  re- 
sistance of  the  machine,  so  that  in  diving  steeply  the  aeroplane  read- 
ily acquires  a  velocity  through  the  air  very  much  greater  than  its  maxi- 
mum high  speed  in  horizontal  flight.  If,  after  acquiring  a  great  speed, 
due  to  a  steep  dive,  the  aeroplane  is  turned,  to  flatten  out  and  fly  hori- 
zontally, a  centripetal  force  must  be  exerted  on  the  wings  in  order 
to  make  the  turn.  For  any  given  radius  of  turn  r,  in  feet,  an  aero- 
plane of  weight  w,  pounds,  having  acquired  a  speed  thru  the  air  of 
v  feet  per  second,  will  have  to  have  exerted  upon  it  a  force  equal  to 
wv2/32.2  r  (see  p.  30).  in  order  to  flatten  out  at  this  rate.  As  an  ex- 
ample of  the  magnitude  of  this  force,  let  us  take  the  case  of  an  aeroplane, 
weight  loaded  =  2000  Ibs.,  which  dived  a  few  hundred  feet  and  ac- 
quired a  speed  of  75  miles  an  hour  (110  f.  p.  s.),  and  which  the  pilot 
rather  quickly  flattens  out  by  turning  up  on  an  arc  of  radius  =  100 
feet  —  a  quick  recovery  to  be  sure,  but  not  at  all  unusual.  The  centri- 
petal force  exerted  on  the  wings,  is, 

wv2       2000  x  12,100 

=  7520  Ibs. 

gr        32.2x100 

a  stress  almost  four  times  as  great  as  the  weight  of  the  machine. 

The  magnitude  of  this  force  for  greater  speeds  and  sharper  turns 
would  seem  enormous,  but  there  is  a  definite  limit,  since,  if  this  force, 
which  makes  the  machine  take  a  curved  path,  exceeds  the  maximum 
pressure  corresponding  to  the  angle  with  the  highest  K  of  the  wing 
surfaces  for  the  particular  aeroplane  speed,  the  aeroplane  will  "slip" 
and  refuse  to  take  this  curve,  since  the  air  pressure  on  its  wings  cannot 
be  made  greater  than  the  maximum  pressure.  It  becomes  quite  easy 
then  to  determine  the  limiting  stress.  The  maximum  speed  attain- 
able on  a  glide  is  the  one  for  which  the  air  resistance  becomes  equal 
to  the  weight  of  the  machine.  This  limits  the  speed  of  falling.  A 
simple  way  to  estimate  it  is  to  determine  from  the  Resistance  Chart, 
the  minimum  value  of  the  quantity  KS  in  R  =  KSV2.  Then  sup- 
plying this  same  K  S,  and  R  =  Weight  of  machine,  a  solution  is  ob- 
tained for  V2,  the  maximum  diving  speed.  Thus,  it  is  found,  p.  96, 
that  at  85  m.  p.  h.,  on  the  Resistance  Chart,  R  =  365  Ibs.,  and  V2  = 
7225;  it  follows  that  K  S  =  365/7225  =  .0505.  The  assumed  total 
weight  is  1800  Ibs.,  so  that 


1800  =  .0505  V2,  and  V  =  V  35,600  =  189  miles  per  hour. 


108 

The  maximum  value  of  K  for  the  wing  (about  .003),  would  indicate 
that  if  the  machine  after  diving  several  thousand  feet  vertically,  could 
suddenly  be  turned  up,  the  wings  would  "bite"  the  air  with  a  force 
KS  V2  =  .003  x  335  x  1892  =  35,650  pounds,  which  is  almost  twenty 
times  the  weight  of  the  machine.  This  is  the  limit  that  is  approached, 
and  it  is  clear  that  the  lower  the  head  resistance  of  a  machine  and  the 
greater  the  surface  and  weight,  the  greater  does  this  become.  On 
the  other  hand,  the  greater  the  longitudinal  moment  of  inertia,  the 
more  difficult  does  it  become  to  flatten  out  sharply. 

In  turning,  the  additional  force  on  the  wing,  caused  by  banking 
the  machine,  and  required  in  order  to  hold  the  machine  to  the  turn, 
may  be  determined  in  the  same  way.  Other  excessive  stresses,  such 
as  those  induced  by  sharp  upward  puffs,  are  not  as  easily  evaluated, 
but  careful  observation  indicates  that  the  forces  of  sharp  puffs,  or 
sudden  changes  in  wind  direction,  may  easily  give  stresses  three  to 
four  times  the  weight  of  the  machine. 

Although  the  stresses  in  the  main  wings  are  the  most  important 
ones,  the  other  parts  of  the  aeroplane  also  are  subjected  to  great  pres- 
sures. The  effects  of  sudden  maneuvers,  or  of  gusts,  in  snapping  the 
tail  around,  not  only  introduce  great  pressures  on  the  tail,  but  subject 
the  fuselage  to  severe  twists.  The  proper  proportioning  of  parts  to 
resist  vibration,  due  to  variations  in  the  engine  and  propeller,  is 
almost  entirely  a  matter  of  experience.  And  the  stresses  introduced 
by  landing  shocks  are  a  separate  class,  requiring  careful  considera- 
tion and  much  experience,  to  be  properly  taken  care  of.  In  taxi-ing 
on  the  ground  on  some  aeroplanes  with  tail  skids,  enormous  twist- 
ing stresses  are  induced  in  the  fuselage,,  by  sharp  turns,  that  every 
careful  pilot  avoids  as  much  as  possible,  since  all  such  stresses  are  un- 
necessarily racking  and  fatiguing  the  aeroplane  structure. 

The  maximum  stresses  in  an  aeroplane  may  become  very  large 
but,  in  the  hands  of  an  expert  pilot,  they  can  be  kept  under  control. 
Supported  in  the  most  perfect  pneumatic  fashion  imaginable,  and 
operated  with  skill  and  caution,  an  aeroplane  is  not  likely  to  receive 
impact  shocks  of  dangerous  magnitude,  and  at  the  present  time  a  break- 
ing strength  of  8  times  the  stresses  due  the  weight,  appears  to  compro- 
mise all  opposing  features  properly  and  to  give  a  sufficient  "safety  factor" 
for  military  purposes. 

Kinds  of  Stresses. 

In  an  aeroplane,  distinction  can  be  made  between  six  different 
kinds  of  stresses: 

(1)  Lift  stresses  on  the  wings  due  to  the  lifting  force  equal  to 
the  weight,  and  carried  by  the  main  struts  and  wires. 


109 

(2)  Drift  stresses  on  the  wings,  taken  account  of  by  the  inte- 
rior cross-bracing  of  the  wing. 

(3)  Stresses  on  the  control  surfaces,  transmitted  thru  the  frame 
or  fuselage  of  the  aeroplane. 

(4)  Stresses  on  various  small  items  due  to  their  air  resistance. 

(5)  Stresses  induced  by  the  pull  or  push  of  the  propeller  and 
secondary  effects  of  gyroscopic  action  or  vibrations  on  the  engine  bed. 

(6)  Landing  stresses  on  the  entire  machine,   due  to  the  shock 
of  alighting.     In  view  of  the  variable  nature  of  landing  fields  and  of 
air  conditions  near  the  ground,  estimates  of  these  stresses  are  difficult 
to  make,  and  are  largely  a  matter  of  experience  for    any   particular 
machine. 

The  thrust  of  the  propeller  is  the  largest  single  air  force  acting 
at  any  point  on  the  machine,  and  necessitates  proper  distribution  over 
the  frame.  But  it  is  definite  in  magnitude,  and  easily  taken  care  of. 

The  consideration  given  stresses  here,  is  not  for  the  purposes  of 
design,  but  rather  to  enable  the  military  aviator  more  readily  to  under- 
stand the  information  on  stresses  supplied  by  the  manufacturer.  The 
most  important  stresses  are  occasioned  by  the  load  lifted  on  the  wing 
structure. 

Stresses   in  the   Wings   and   Bracing. 

Since  the  consideration  and  method  of  determining  the  lifting 
stresses  in  the  main  supporting  wings  may  be  extended,  readily,  to 
other  stresses  in  the  machine,  it  may  prove  beneficial  to  take  up  an 
example. 

The  process  of  determining  stresses  consists,  of 

(1)  Finding  what  proportion  of  the  load  is  carried  by  different 
parts  of  the  frame ; 

(2)  Determining  what  stresses  these  loads  induce  in  the  mem- 
bers of  the  framework. 

Since  a  biplane  involves  practically  every  feature  requiring  con- 
sideration, we  may  take  as  an  example  the  aeroplane  assumed  in  Chap- 
ter VIII,  in  which  the  full  load  weight  is  1800  Ibs.,  the  surface  area  335 
sq.  ft.,  chord  5  ft.,  gap  5  ft.,  and  span  36  feet.  Let  us  assume  that  the 
bracing  is  of  the  familiar  strut  and  cross-wire  type  usually  termed  a 
"Pratt  Truss." 


110 

REAR   TRUSS  CFRONT 


t/    trass  =      IJ4&  *   body 


MOMENT   DIAGRAM 


HALF  SPAM  Of  AfROPVWE    *  ^8  FT. 

STRESS  ANALYSIS   FOR    BIPLANE   TRUSS 

The  loads  at  the  connecting  points  U,  U',  U",  called  panel  points,  are  indicated  on  the  diagram, 
and  are  due  to  the  air  load  on  the  wings.  The  heavy  line  wires  are  the  "flying  wires,"  taking  the  stresses 
due  to  these  loads;  and  the  dashed  lines  in  the  truss  diagram  are  the  "landing  wires,"  taking  the  weight 
of  wings  on  landing. 

In  the  graphical  stress  method,  each  panel  point  is  considered  in  order,  and  for  each  one  a  closed 
triangle  or  polygon  of  forces  is  drawn.  The  force  polygons  must  all  close,  since  the  point  is  in  equili- 
brium. 

First,  a  "sense"  of  rotation  for  the  diagram  is  chosen  and  indicated  by  the  arrow  as  clockwise, 
and  a  scale  to  which  to  lay  off  the  forces  is  chosen.  Then,  on  the  truss  diagram,  the  regions  between 
forces  are  lettered  A,  B,  C,  etc.,  the  forces  considered  being  only  the  forces  carrying  the  truss  load.  That 
is  why  the  compression  in  L"U"  is  not  considered,  since  it  is  carried  to  U"  and  from  there  over  the  truss. 
Taking  the  first  point  U",  we  have  the  force  between  A  and  B,  called  ab,  =  205  Ibs.  total,  and  the  force 
of  compression  in  U"U',  called  be  and  a  third  force,  the  tension  in  the  wire  ca.  Thus,  there  are  three 
forces  at  this  point.  The  magnitude  of  one  is  known  and  the  direction  of  the  other  two,  so  that  a  force 
triangle,  as  given  on  the  stress  diagram  abc,  may  be  drawn,  such  that  ab  =  205  Ibs.  to  scale,  be,  is  parallel 
to  U"U',  and  ac  is  parallel  to  U"L'.  Their  point  of  intersection  establishes  the  closing  point  of  the  tri- 
angle, thus  determining  ac  and  be  in  Ibs.,  merely  by  reading  their  lengths  to  the  same  scale  to  which  ab 
was  laid  off. 

Panel  point  L'  is  now  taken,  the  forces  being  taken  in  the  same  order  going  around  the  point  clock- 
wise. First,  we  have  ac,  already  solved  and  then  cd,  the  strut  compression,  the  direction  of  which  we 
know,  so  we  draw  cd  thru  c,  parallel  to  U'L'.  To  obtain  the  rest  of  the  polygon  it  is  now  necessary  to 
consider  ea,  acting  upwards  at  L',  which  is  laid  off  on  the  vertical,  and  then  to  close  the  polygon  the  other 
force  line  de,  may  be  drawn  thru  e,  parallel  to  L'L,  the  point  d  being  located  by  the  intersection  of  the 
lines  of  action  of  the  two  unknown  forces  thru  e  and  c.  Thus,  with  d  found,  cd  and  de  are  readily  read 
to  scale. 

A  similar  process  is  employed  for  the  rest  of  the  truss. 


Ill 

A  moment's  thought  on  the  manner  in  which  the  air  force  on  the 
wings  lifts  the  rest  of  the  machine,  will  lead  to  the  simple  conception 
that  an  aeroplane  is  virtually  a  swing  bridge,  turned  upside  down, 
with  a  uniform  static  load  of  the  simplest  kind,  equal  in  average  in- 
tensity to  1800/335  =  5.4  Ibs.  per  sq.  ft.  (a  factor  often  termed  the 
"loading"  of  the  wing).  The  complicated  stress  determinations  for 
steel  bridges  resulting  from  "live  loads,"  such  as  moving  locomotives 
of  300,000  Ibs.  weight,  are  happily  in  another  realm,  and  as  for  the 
actual  consideration  of  the  aeroplane  structure  itself,  it  is  well  to  real- 
ize that  it  is  the  simplest  kind  of  a  bridge. 

For  the  purposes  of  this  example  reference  is  made  to  only  one- 
half  of  the  machine,  since  the  other  side  is  symmetrical,  and  it  fol- 
lows that  the  upper  and  lower  wings  under  consideration  together  carry 
half  the  load. 

The  load  actually  carried  by  the  structure  is  the  total  weight  less 
the  weight  of  the  wings  themselves,  since  the  latter  pressing  down 
by  gravity  directly  against  the  air  pressure,  relieve  the  struts  and  wires 
of  having  to  transmit  any  stresses  due  to  their  weight.  If  the  weight 
of  the  wings  is  taken  at  240  Ibs.  —  a  reasonable  figure  —  the  load  on  the 
side  of  the  machine  we  are  considering  equals  (1800  —  240)  -*-  2  =  780 
Ibs.  This  is  the  distributed  load  over  the  upper  and  lower  wings.  But, 
due  to  the  biplane  effect  (Chap.  VII),  the  upper  wing  may  be  expected 
to  carry  a  considerably  greater  proportion  of  this  load.  In  general, 
on  a  biplane  the  upper  plane  carries  about  60  %  of  the  load  and  the 
lower  plane  40  %. 

From  this  it  is  indicated  that  the  upper  plane  on  one  side,  carries 
780  x  .60  =  468  Ibs.,  whereas  the  corresponding  lower  plane  carries 
312  Ibs. 

This  load  is  transmitted  by  the  cloth  covering  to  the  ribs,  each 
one  of  which,  acting  as  a  beam,  transmits  the  load  to  the  spars,  which 
in  turn  are  suitably  braced  to  the  body  by  struts  and  wires,  so  that 
Ibs.  weight  in  the  body  are  carried  by  Ibs.  per  sq.  ft.,  air  pressure  on 
the  outstretched  wings.  But,  since  this  is  distributed  between  the 
spars,  of  which  in  this  case  (see  p.  110)  there  are  two,  it  follows  that 
separate  stress  determinations  must  be  made  for  the  front  and  rear 
truss.  This  at  once  necessitates  determining  what  portion  of  the 
load  each  spar  carries. 

The  position  of  the  center  pressure  determines  this  readily,  for 
if  the  c.  p.  were  midway  between  the  two  spars,  obviously  they  would 
each  carry  half  the  load,  and  if  the  c.  p.  were  directly  in  line  with  a 
spar,  the  entire  load  on  the  wing  would  be  carried  by  it.  Since  the 
c.  p.  moves,  and  we  are  here  interested  in  the  maximum  stresses  due 
to  carrying  the  weight,  the  next  step  is  to  determine  the  max.  rear 


112 

position  of  c.  p.  applying  the  greatest  load  to  the  rear  spar,  and  max. 
front  position  for  the  front  spar.  This  is  done  (p.  110),  and  from  what 
information  we  already  have  on  the  aerofoils  and  the  aeroplane,  we 
may  recall  that  the  former  condition  corresponds  to  a  high  speed  and 
low  angle  of  incidence,  and  the  latter  to  a  slow  speed  and  high  angle  of 
incidence. 

Since  the  data  indicates  that  the  rear  spar  carries  a  maximum 
of  75  %  of  the  load  at  0°  incidence,  it  follows  that  the  upper  plane  rear 
spar,  which  spans  16.5  feet,  carries  (468  X  .75)  -^  16.5  =  21.3  Ibs. 
per  foot  run,  and  the  lower  plane  rear  spar,  carries  (312  x  .75)  -r-15.5 
=  15.1  Ibs.  per  foot  run,  —  the  spans  being  taken  to  include  allow- 
ances for  the  rake  and  reduction  of  pressure  of  the  ends  of  the  planes, 
and  for  the  body  section. 

Knowing  the  spans  we  can,  as  has  been  done  on  p.  110,  indicate 
the  load  at  each  panel  point  U,  U',  U",  etc.  This  load,  which  is  the 
force  carried  thru  the  truss,  results  from  the  uniform  loads  on  adja- 
cent spans.  For  example,  U'  carries  half  the  load  on  span  UU'  =  21.3 
X  3.125,  plus  half  the  load  on  span  U'U"  =  21.3  x  3.625,  which  to- 
gether give  144  Ibs.  The  other  panel  loads  are  obtained  in  the  same 
way,  and  since  the  slopes  of  wires  and  depth  of  truss  are  outlined  to 
scale,  the  graphical  method  explained,  p.  110,  is  reaily  made  use  of 
to  determine  the  stresses  in  the  members  of  the  truss. 

The  tension  stress  on  any  wire,  as  determined  in  the  stress  dia- 
gram, may  be  compared  directly  with  the  breaking  strength  of  the  wire, 
to  determine  the  safety  factor.  Thus,  if  L  U'  indicated  by  dg,  as  having 
a  stress  of  730  Ibs.,  consists  of  two  5/32"  cables  each  with  a  breaking 
strength  of  3000  Ibs.,  the  "safety  factor"  is  more  than  8. 

The  strength  of  struts  is  not  as  readily  found,  since  struts  usual- 
ly fail  by  bending.  Only  in  the  case  where  a  strut  is  very  short  and 
thick  is  it  possible  to  find  its  strength  by  multiplying  the  compres- 
sion strength  of  the  material  in  pounds  per  square  inch  by  the  cross- 
sectional  area.  Failure  from  bending  makes  it  necessary  to  introduce 
standard  engineering  formulae*,  which  vary  greatly  among  them- 
selves and  are  largely  based  on  experiment.  Their  object  is  merely 
to  determine  a  reduced  value  of  the  allowable  compressive  strength 
of  the  material,  to  take  into  account  the  weakening  due  to  bending. 
As  an  example,  spruce,  ordinarily,  stands  5600  Ibs.  per  sq.  inch  in  direct 
compression,  whereas  one  of  the  most  practical  strut  formulae  taking 
into  account  the  average  dimensions  of  aeroplane  struts,  reduces  this 

*  These  formulae  and  data  are  ordinarily  furnished  by  the  manufacturer,  and 
if  need  be  are  readily  checked  by  actual  breakage  test  on  a  strut.  A  typical  formula 
is  the  RAF  strut  formula. 

FA 

Crippling  Strength  = ,  in  which  F  =  allowable  compression  stress, 

1  +  6500  P/k 
A  =  area  of  section  1  =  length  of  strut  in  inches,  and  k  =  least  radius  of  gyration. 


113 

to  about  l/5th,  giving  1100  Ibs.  per  sq.  in.,  as  the  ultimate  strength 
to  be  expected.  If  U'L'  is  made  of  spruce,  with  2.3  sq.  in.  cross  section, 
it  may  be  expected  to  have  a  strength  of  about  2.3  x  1100  =  2500  Ibs., 
and  since  the  stress  induced  is  310  Ibs.,  there  is  a  safety  factor  of  8  (see 
p.  110). 

Spars. 

The  stresses  on  the  wing  spars  are  considerably  more  compli- 
cated and  frequently  of  greater  importance,  than  stresses  on  other 
members.  In  almost  all  aeroplanes,  nowadays,  the  upper  spars  are 
the  weakest  structural  parts. 

This  is  due  largely  to  their  receiving  a  combination  of  stresses 
which,  as  will  be  seen  later,  causes  the  spar  progressively  to  weaken  as 
the  stresses  increase,  due  to  deflection. 

The  type  of  construction  of  wings,  is  now  almost  universally  stand- 
ardized, and  consists  of  carrying  the  air  pressure  by  means  of  cloth 
covering  to  light  ribs  running  fore  and  aft,  which  are  formed  to  give 
the  aerofoil  section  desired.  These  ribs  are  carried  by  large  beams 
or  spars  running  across  the  wing  transversely,  and  these  spars  are  braced 
to  the  rest  of  the  machine  by  suitable  struts  and  wires,  as  already  indi- 
cated. The  stresses  on  the  spars,  therefore,  may  be  divided  into  two 
items : 

(1)  The  stresses  due  to  the  loading  of  the  spar  as  a  beam,  carry- 
ing the  air  pressure  loads  transmitted  by  the  wing  covering  and  ribs; 

(2)  The  stresses  due  to  their  part  in  the  general  bracing  of  the 
wing  truss,  as  found  by  the  stress  diagram,  p.   110;  which  indicates 
at  once  that  as  members  of  the  rigid  truss,  the  lower  spars  are  sub- 
jected to  tension  and  the  upper  spars  to  compression. 

The  result  of  the  application  of  these  stresses  to  the  spar,  may 
be  taken  up  as  follows : 

(a)  Compression   or   Tension   Stress   in    Spar.  —  The   allowable 
breaking  load  in  Ibs.  per  sq.  in.,  for  the  particular  material  used,  mul- 
tiplied by  the  area  of  the  cross  section  of  the  spar  in  sq.  inches,  gives 
the  breaking  strength,  which,  divided  by  the  load  as  determined  from 
the  stress  diagram,  determines  the  factor  of  safety  for  that  stress. 

(b)  Bending  due  to  the  pull  of  wires  of  the  frame,  attached  un- 
symmetrically  with  reference  to  the  neutral  axis  of  the  spar.     This 
feature  on  some  machines  is  of  considerable  magnitude,  but  fittings 
are  so  readily  made  to  bring  the  pull  of  wires,  etc.,  all  together  at  any 
one  point,  symmetrical  with  the  beam's  center  line,  that  they  should 


114 


UNIFOKH     LOAV 
(VNCfHTKAri-t  LOW  W  Iba  .  per  foel  *vn 


. '.  73  /7/v!f>  r, 

OF  7»f   SFCTION    BY 


Bending  Moments  and  Sections  of  Beams 


always  be  demanded,  so  as  to  enable  this  unnecessary  load  on  the  wing 
to  be  eliminated. 

(c)  Bending  and  Compression  due  to  the  Drift  Load.  —  This  is 
an  element  in  the  rigidity  of  the  wing,  which  requires  that  the  stresses 
be  taken  care  of  by  suitable  cross  bracing,  etc.,  but  as  a  factor  in  de- 
termining the  strength  of  the  spars,  the  drift  loads  are  so  small  in  pro- 
portion to  the  lift  loads,  that  they  are  negligible,  for  our  purposes. 

(d)  Bending  due  to  Uniform  Load  of  air  pressure  on  the  Wing. 
-  This  load  is  the  principal  one  on  a  long  span,  and  since  the  load  may 

be  considered  as  spread  uniformly,  along  the  spar,  the  ordinary  engi- 
neering formulae  for  beams  are  directly  applicable. 

(e)  But  the  bending  of  the  spar  due  to  the  uniform  air  loading, 
introduces  a  certain  deflection  of  the  beam,  which  gives  any  compres- 
sive  force  on  the  spar  due  to  the  truss  load  a  chance  still  further  to 
increase  the  bending  moment.     In  the  inner  spars  of  the  upper  wing 
of  an  aeroplane,  this  stress  is  by  no  means  a  negligible  one. 

The  stresses  on  a  beam,  then,  are  first  considered  in  the  deter- 
mination of  the  several  bending  moments  due  to  the  loading  and  these 
are  combined  and  charted  for  convenience  on  a  "moment  diagram," 
an  example  of  which  is  given  on  p.  110.  The  truss  is  laid  off  to  scale 
as  indicated,  and  the  bending  moment  values  are  given  in  "Ibs.  ft." 
Suitable  corrections  are  applied  for  the  continuity  of  the  spars,  and 
these  diagrams,  furnished  for  each  spar  by  the  manufacturer,  enable 
the  value  of  the  total  bending  moment  at  any  point  to  be  read. 


115 

Bending   Moment. 

It  would  prove  beneficial  here  to  consider  what  is  meant  by  "bend- 
ing moment,"  and  how  it  is  made  use  of  in  strength  determinations. 

In  the  tension  on  wires  and  the  compression  on  struts  or  spars, 
the  load  stresses  are  taken  up  by  members  which  have  areas  of  a  cer- 
tain number  of  square  inches  of  a  certain  material.  It  is  known  by 
experiments  that  the  particular  manner  in  which  the  material  is  used 
permits  of  assigning  to  it  a  certain  breaking  strength,  called  "fibre 
strength"  or  "modulus  of  rupture,"  which  is  most  easily  expressed 
as  a  certain  number  of  "Ibs.  per  sq.  inch."  The  area  of  the  member 
times  the  strength  of  its  material  per  unit  of  area,  gives  the  total  act- 
ual force  that  it  is  reasonable  to  expect  would  break  the  member  in 
question.  In  beams,  however,  the  loads  are  not  applied  endwise, 
so  that  instead  of  having  a  direct  push  or  pull,  the  beam  is  subjected  to  a 
bending. 

The  loads  on  the  beam  tend  to  make  it  sag  and  the  amount  of  sag 
for  any  given  load  is  determined  not  only  by  the  load,  but  by  the  man- 
ner in  which  the  beam  is  supported. 

1.  The  beam  may  merely  be  resting  freely  on  its  supports,  or 
pinned  to  them  —  in  which  case  it  is  termed  a  "simply  supported" 
beam. 

2.  The  beam  may  be  fixed  at  both  ends  and  held  firmly  in  its 
supports,  or  may  be  continuous  over  several  spans  —  in  which  case  it 
is  termed  a  "fixed  end"  beam.     A  "cantilever"  is  a  fixed  end  beam,  held 
only  at  one  end. 

When  loaded,  the  beam  resists  the  bending  tendencies  of  the  load 
with  a  force  which  varies  from  that  of  a  light  string  (which  has  practi- 
cally no  beam  strength)  to  the  deep  plate  girders  of  a  railroad  bridge,  and 
which  is  determined  by  the  shape,  span,  size,  and  material  of  the  beam. 

The  mechanics  of  the  action  of  a  beam  are  very  simple.  The 
forces,  and  air  pressure  loads,  have  lever  arms  and,  therefore,  moments 
about  any  point  of  a  spar  that  we  care  to  consider.  These  can  all 
be  summed  up  into  an  equivalent  force,  in  Ibs.,  with  a  certain  lever 
arm  in  feet.  This  moment  is  the  "bending  moment"  for  the  partic- 
ular point  under  consideration.  On  beams  that  are  loaded  uniformly, 
like  the  spars  of  an  aeroplane,  the  maximum  bending  moment  is  found 
at  the  center  of  the  beam,  and  decreases  as  the  points  of  support  are 
approached,  with  the  exception  that  the  continuity  of  spars  over  two 
or  three  spans  may  slightly  modify  this. 

This  maximum  bending  moment  for  any  beam,  loaded  uniformly, 
is  readily  found  by  supplying  values  for  the  quantities  in  the  accom- 
panying simple  formulae.  This,  then,  gives  us  the  value  of  the  "bend- 


116 

ing  moment"  due  to  the  air  loading,  which  is  the  important  one  for 
the  spar,  but  which  is  slightly  modified  by  the  other  forces  causing 
bending,  as  already  indicated. 

The  total  maximum  bending  moment  as  determined  by  the  manu- 
facturer and  read  from  the  diagram  for  the  beam,  is  equal  in  its  effect 
to  the  twist  of  a  force  in  Ibs.  with  a  leverage  in  ft.,  giving  the  same 
Ibs.  ft.  moment,  about  the  center  of  the  section  of  the  beam,  at  the 
point  considered. 

Any  "twist"  or  moment  of  this  kind  would  naturally  be  taken 
up  by  a  tension  resistance  on  the  upper  side  of  the  beam  and  a  com- 
pression on  the  lower.  The  final  test  is  what  "the  extreme  fibre"  of 
the  beam  will  stand,  since,  if  the  beam  begins  to  cripple  on  the  upper 
or  lower  flange,  it  will  progressively  weaken  to  the  breaking  point. 

The  strength  of  the  extreme  fibre  of  a  beam,  then,  expressed  in 
Ibs.  per  sq.  inch,  will  give  us  a  measure  of  the  resisting  force  of  the 
beam;  and  the  depth  of  the  beam  from  the  center  or  neutral  axis  to  the 
outer  edge  is  the  lever  arm  of  this  "Resisting  Moment"  of  the  beam, 
which  opposes  the  "Bending  Moment"  of  the  loads. 

This  Resisting  Moment  for  any  beam  is, 

M  =  K  I/d 

where,  K  =  the  strength  per  sq.  inch  of  the  material,  I  =  the  moment 
of  inertia  of  the  section,  and  d  =  the  distance  from  the  neutral  axis 
to  the  extreme  fibre  =  ^  depth  of  beam. 

If  it  were  desired  to  know  what  fibre  stress  was  induced  in  a  beam, 
for  which  I  and  d  were  known,  by  a  bending  moment  M,  the  value 
of  which  is  known,  it  would  merely  be  necessary  to  solve  for  K  in  the 
above  formula.  So  that  on  a  spar,  if  to  this  determined  fibre  stress 
there  is  added  the  stress  per  sq.  in.,  due  to  the  compression,  a  value 
is  at  once  obtained  for  the  total  intensity  of  stress  in  Ibs.  per  sq.  in., 
on  the  weakest  extreme  fibre  of  the  beam.  Comparing  this  value  with 
the  breaking  strength  of  the  material  in  Ibs.  per  square  inch  gives  the 
safety  factor  for  the  spar. 

An  example  would  serve  to  illustrate  how  the  safety  factor  of  a 
spar  may  be  determined.  Let  us  suppose  that  the  rear  spar  in  the 
span  UU',  (see  p.  110)  consists  of  a  rectangular  section  beam  of  spruce 
3  inches  deep  and  1  inch  wide,  and  with  a  cross-sectional  area  of  3  sq. 
inches.  The  span  1  is  6  1/4  ft.  and  the  load  per  foot  of  spar  is  w  =  21.3 
Ibs.  per  foot. 

We  could  read  from  the  moment  diagram  furnished  what  the 
maximum  bending  moment  is,  account  having  been  taken  of  the  nature 
of  fixing  of  the  ends  of  the  spar,  the  moments  due  to  any  unsymmetrical 
wire  pulls,  etc.  For  the  purposes  of  convenient  analysis,  in  the  field,  how- 
ever, it  is  not  necessary  to  go  into  these  details.  A  sufficiently  accurate 


117 

conception  of  the  bending  moment  in  the  beam  is  obtained  by  con- 
sidering the  ends  fixed,  and  finding  the  large  moment  due  solely  to 
the  air  load  on  the  spar  per  foot  run.  The  table  given  shows  this  to  be, 

wl2       21.3x6.252 

B.M.  =  -  -  =  69.5  Ibs.  ft. 

12  12 

It  is  desired  to  find  what  stress  this  bending  moment  induces  on 
the  outer  fibre  of  the  beam.  We  merely  substitute,  then,  in  the  equa- 
tion, 

M  =  K  I/d 

taking  care,  however,  to  express  M  in  Ib.  inches,  by  multiplying  by  12, 
to  correspond  with  the  units  of  I  and  d. 

For  any  rectangular  section  beam,  the  moment  of  inertia  I,  is 
bd3/12,  and  in  this  case  d  =  3  inches  and  d  =  1  inch,  so  that  I  =  27/12  = 
2.25.  The  distance  to  extreme  fibre  from  the  neutral  axis  equals  half 
the  depth  of  the  beam  =  1  }/£  inches,  and  solving  we  get 

2.25 

M  =  69.5  x  12  =  K  x ,  whence 

1.5 

K  =  555   Ibs.   per   sq.   inch. 

To  this  stress  must  be  added  that  due  to  the  compression  truss 
load  — •  also  carried  by  these  same  fibres.  From  the  diagram  on  p.  110, 
this  is  seen  to  be  870  Ibs.  and  being  distributed  over  the  3  sq.  inches  of 
cross-section  of  the  spar,  adds  a  stress  of  290  Ibs.  per  sq.  in.  to  the  spar. 

The  totaf  fibre  stress,  then,  is  the  sum  =  845  Ibs. 

The  material  of  the  spar  being  spruce,  which  has  a  fibre  strength 
of  5600  Ibs.  per  sq.  inch,  it  follows  that  the  safety  factor  for  the  spar 
is  5600  +  845  =  6.64. 

It  is  of  interest  to  note  that  the  continuity  of  spars  over  several 
spans  is  apt  to  reduce  the  value  of  the  max.  bending  moment,  but  in- 
creases its  value  at  the  points  of  support.  This  is  determined  by  the 
Theorem  of  Three  Moments,  which  it  is  not  necessary  to  consider 
here,  but  a  characteristic  bending  moment  diagram  inclusive  of  these 
corrections  is  shown,  p.  110.  As  indicated  in  this  example  the  b.  m., 
due  to  the  air  load  on  the  span,  gives  an  excellent  and  sufficiently  intel- 
ligible indication  of  the  magnitude  of  the  stress  in  the  spar. 

The  upper  rear  spar  of  the  panel,  next  to  the  body,  on  a  tractor 
biplane  is  almost  always  the  weakest  member  of  the  entire  structure, 
and  is  subjected  to  a  combination  of  loads  that  are  very  formidable. 


118 

A  study  of  a  stress  diagram,  as  to  the  distribution  of  loads,  and 
the  magnitude  of  bending  moments,  should  always  be  made  by  a  con- 
scientious aeroplane  pilot,  in  order  to  obtain  an  appreciation  of  the 
nature  of  the  stresses  his  machine  is  required  to  withstand. 

The  weakening  of  spars  by  the  drilling  of  holes,  for  some  extra 
kind  of  fitting,  should  never  by  done,  until  the  moment  diagrams  have 
been  consulted  and  a  rough  calculation  made  of  how  much  the  reduc- 
tion in  sectional  area  caused  by  the  hole,  is  going  to  weaken  the  spar. 

The  splicing  and  re-enforcing  of  spars  by  ferrules,  etc.,  is  taken 
up  later,  and  should  always  be  considered  in  the  light  of  preserving  depth 
of  section  and  strength  in  extreme  fibre. 

Tightening  of   Wires. 

A  feature  that  results  directly  from  a  consideration  of  the  aero- 
plane structure  as  a  truss,  is,  that  extra  stresses  may  be  induced  on 
the  members  by  tightening  up  too  much  on  some  parts,  lack  of  proper 
fitting,  etc.  The  systems  of  wiring  on  aeroplanes  consist  of  the  "fly- 
ing wires,"  indicated  by  full  lines  on  p.  110,  and  the  "landing  wires," 
indicated  by  the  dashed  lines.  The  stresses  for  the  former  are  deter- 
mined by  the  methods  already  outlined. 

The  stresses  on  landing  wires  are  largely  indeterminate,  and  proper 
strength  to  take  landing  shocks  is  a  matter  of  experience.  Due  to  the 
possibility  of  large  negative  air  loads,  the  "landing  wires"  are  usually 
made  of  practically  the  same  strength  as  the  "flying  wires." 

In  addition  to  these,  the  general  rigidity  of  the  truss  and  resist- 
ance to  drift  loads,  twists,  etc.,  requires,  cross  wiring  from  front  to 
rear  of  the  panels.  (See  photographs  in  Chap.  II.) 

The   entire    structure,    therefore,    is    cross   wired    and    completely 
braced,  although  in  flying  only  the  "flying  wires"  should  take  the  loads. 

Nevertheless,  complicated  extra  loads  can  be  induced  on  the  spars 
and  struts  and  flying  wires  by  the  universal  mistake  of  having  the 
wires  too  tight.  Thus,  if  L  U'  and  U  L'  are  both  tightened  up  too 
much,  L  U'  before  it  ever  receives  its  proper  flying  load,  is  carrying 
an  initial  load  due  merely  to  the  tightening,  while  the  spars  L  L'  and 
U  U'  are  perhaps  already  bent  up  and  weakened  before  they  ever  re- 
ceive their  air  load  bending  moments.  Buckling  of  spars  and  struts 
and  initial  stresses  in  wires,  due  to  having  the  trusses  tightened  up 
too  much,  greatly  fatigue  the  parts,  introduce  entirely  uncalled  for 
stresses  and  are  apt  to  result  in  serious  crippling.  Wires  should  never 
"sing"  and  need  only  be  tight  enough  to  avoid  deflection  of  the  truss 
when  loaded. 


119 
"Follow  Thru" 

The  characteristic  wing  sections,  struts  and  wires,  the  stresses 
in  which  have  been  considered  here,  are  readily  distinguishable,  in 
the  photographs  of  aeroplanes,  p.  15  and  p.  17,  and  may  conveniently 
be  referred  to.  Some  aeroplanes  have  "overhangs,"  others  more  panels 
than  taken  in  the  example  p.  110,  etc.,  but  the  general  principles  of 
finding  the  air  loads  and  solving  the  stresses  graphically  are  the  same. 

There  is  one  very  important  feature,  however  obvious  it  maybe 
on  the  stress  diagram,  that,  to  the  unpracticed  eye,  is  not  so  easy  to 
appreciate  on  a  full  sized  aeroplane,  i.  e.,  the  degree  in  which  the  stresses 
induced  in  the  wires,  struts  and  spars,  are  carried  thru  the  truss  to 
their  logical  end,  so  as  really  to  "complete"  their  strength.  A  wire 
may  be  strong  enough  in  itself  to  hold  the  stress  induced  in  it,  but  the 
fitting  holding  this  wire  at  the  base  of  the  strut  may  not  be  properly 
proportioned. 

On  monoplanes,  (see  p.  20),  the  wing  spars  are  subjected  to  a 
large  compression,  due  to  the  truss  load,  and  may  be  made  strong  enough. 
These  spars,  however,  on  either  side*  of  the  body,  press  against  the 
body  towards  each  other  with  an  enormous  compression.  Lack  of 
attention  in  following  thru  these  stresses  so  that  the  spars  could  butt 
directly  against  each  other  with  ample  compressive  strength,  led  many 
constructors  to  provide  therefor  merely  by  permitting  the  spars  to 
rest  in  sockets  against  the  body  —  with  no  suitable  provision  across  the 
body  at  this  point.  Many  accidents  are  attributable  to  the  crush- 
ing of  the  body  by  this  spar  compression,  due  solely  to  lack  of  "follow 
thru." 

A  typical  example  is  found  today  in  more  or  less  serious  measure 
on  many  tractor  biplanes  of  reputable  construction.  Reasons  of  sim- 
plicity and  convenience  in  the  chassis  have  eliminated  auxiliary  safety 
wires  from  points  like  L'  (see  diag.  p.  110)  to  the  chassis.  It  follows, 
then,  that  the  pull  of  the  wire  L  U',  and  the  tension  in  the  spar  L  L', 
are  all  exerted  at  the  point  L,  on  the  body.  It  is  customary  to  draw 
the  diagrams  and  determine  the  stresses  and  safety  factors  for  all  the 
struts,  wires  and  spars,  but  not  always  is  the  proper  attention  given 
to  the  cross  member  at  the  body,  indicated  in  the  diagram  by  O  Y. 
As  a  matter  of  fact,  this  member  carries  an  enormous  tension  —  a  stress 
of  870  Ibs.,  from  both  sides  of  the  truss  —  and  the  "following  thru" 
of  the  tension  in  wire  L  U',  denoted  as  dg,  across  under  the  body,  con- 
necting to  the  wire  symmetrical  to  L  U',  on  the  other  side  of  the  aero- 
plane, is  of  the  very  greatest  importance.  A  safety  factor  of  at  least 
ten  should  be  demanded  on  this  tension  stress,  and  more  attention  paid 
to  it.  Similar  instances  can  be  cited,  but  the  general  principle  is  the 
same,  and  applies  equally  in  importance  to  the  proportioning  of  bolt 
heads,  plate  fittings,  pins  and  turnbuckles  to  develop  the  full  strength 


120 

of  the  wire  or  cable  to  which  they  are  attached.  An  expert  can  spot 
these  flaws  in  construction  quite  readily,  but  the  location  of  the  "weak 
link"  in  the  chain  is  not  always  so  apparent  to  the  amateur,  and  mili- 
tary aviators  can  profitably  spend  considerable  effort  in  acquiring  that 
"knack"  that  will  enable  them  to  locate  lack  of  "follow  thru,"  in  the 
construction  of  the  machines  they  are  using. 

Details  of  construction  that  bear  directly  on  this  are  studied  in 
the  next  Chapter. 


An  aeroplane  with  a  "safety  factor"  of  12  throughout.     The  Sturtevant  military 
tractor,  in  which  provision  is  made  for  carrying  two  gun  turrets. 

Above  —  Views  in   flight   with   and   without   turrets. 


CHAPTER    X. 
ASSEMBLY  AND    CONSTRUCTION. 

Although  it  is  difficult  to  give  in  written  form  all  the  practical 
information  and  directions  desirable  relative  to  the  assembly,  align- 
ment and  verification  of  construction  of  aeroplanes,  a  few  notes  are 
presented  here,  accompanied  by  some  data  on  the  strength  of  aero- 
plane parts,  that  may  be  of  use.  Structural  details  on  aeroplanes 
differ  greatly,  but  the  ones  chosen  here  as  examples  will  serve  to  illus- 
trate the  mode  of  procedure  in  considering  these  details,  and,  at  the 
same  time,  will  be  found  to  give  many  suggestions,  to  help  in  repair 
and  maintenance  work  in  the  field  —  where,  as  already  stated,  resource- 
fulness in  keeping  the  equipment  in  operation  is  of  the  greatest  import- 
ance. 

Aeroplanes  for  other  purposes  may  become  elaborate  in  construc- 
tion and  exceedingly  replete  in  extra  fittings,  but  for  military  pur- 
poses it  is  quite  certain  that  the  structural  details  will  become  as  simple 
and  as  easy  to  repair  as  possible,  with  particular  attention  paid  to  hav- 
ing parts  accessible  for  inspection  and  easy  to  take  down  or  assemble. 
And  in  order  to  reduce  the  amount  of  stores  necessary  to  carry  around 
in  the  way  of  "spare  parts,"  it  should  be  an  elementary  policy  of  the 
construction  department  of  a  Flying  Corps  to  standardize  as  many  parts 
as  possible,  accentuating  interchangeability  of  parts,  and  reducing 
to  the  minimum  the  different  grades  and  thicknesses  of  lumber,  the  dif- 
ferent sizes  of  wire  and  the  thickness  of  steel  plate  used  in  fittings,  so 
that  a  small  stock  of  raw  material  may  be  found  suitable  for  repairing 
practically  any  part  of  the  machine. 

Unpacking. 

An  aeroplane  received  from  the  manufacturer  almost  always  has 
suffered  from  shipment  or  packing  in  one  way  or  another,  and  in  taking 
the  parts  out  of  the  boxes  and  crates,  great  care  should  be  exercised  not 
to  do  any  more  damage. 

Aeroplanes  do  not  seem  so  fragile  when  they  are  all  assembled,  tight- 
ened up  and  trim,  but  when  dis-assembled  they  can  easily  be  maltreated. 

One  of  the  most  serious  things  to  watch  out  for  is  the  bending  or 
twisting  of  wires  and  cables,  as  they  are  coiled  or  uncoiled  for  conven- 
ience. Cable  can  readily  be  unravelled,  and  hard  wire,  if  bent  up  too 
much,  should  under  no  circumstances  be  straightened,  but  the  entire 
wire  must  be  replaced.  The  same  holds  true  of  turnbuckles,  fittings 
and  bolts,  which,  in  unpacking  and  setting  up  may  become  more  or  less 
seriously  bent  up  or  knocked  out  of  true,  and  the  wilful  straightening 


122 

of  these'parts,  without  bringing  them  to  the  attention  of  someone  who  is 
competent  to  judge  of  the  degree  of  weakness  resulting  from  the  damage 
is  most  reprehensible. 

Among  other  things,  it  is  the  universal  experience  that  wings  or 
other  surfaces  may  have  suffered  a  few  holes  or  rips.  These  should  all 
be  repaired,  first  by  cross-stitching  and  then  by  covering  with  a  glued 
patch  of  the  same  material  as  the  wing  covering  —  and  then, 

Care  should  be  taken,  never  to  lay  tools  on  the  planes. 

Caution  should  be  exercised,  not  to  permit  bolts  or  turnbuckles 
to  fall  on  the  ground  or  in  the  sand,  with  possibilities  of  unnecessary 
damage  to  the  threads  by  grit.  And  to  prevent  any  chance  of  loss  or 
error,  each  part  should  be  tagged  and  tied  to  the  place  to  which  it  belongs. 

Alignment,   or   "Trueing  up." 

From  a  consideration  of  the  foregoing  we  are  at  once  led  to  the 
study  of  the  trueing  up,  or  lining  up,  of  the  truss  of  an  aeroplane,  so 
as  to  obtain  the  proper  alignment  of  the  members,  with  respect  to 
each  other  and  to  the  rest  of  the  machine. 

"Trueing  up"  may  be  defined  as  the  process  by  which  the  wings 
and  rudders  are  adjusted  to  the  body  and  line  of  thrust,  so  as  to  give 
the  proper  angle  of  incidence,  dihedral,  decalage,  etc.,  with  perfect 
symmetry. 

Since  slight  errors  in  alignment  cause  marked  changes  in  flying 
qualities,  this  subject  is  a  particularly  important  one  for  practical 
field  work,  and  it  should  be  borne  in  mind  that  an  aeroplane  must  be 
properly  trued  up,  just  like  any  other  delicate  piece  of  machinery, 
before  the  best  results  can  be  obtained  from  it. 

Aeroplanes  can,  of  course,  be  flown  when  more  or  less  out  of  align- 
ment, but  tricky  characteristics  are  apt  to  arise  from  this,  and  the  ma- 
chines will  not  give  their  best  and  most  pleasing  performances,  while 
they  may  actually  prove  dangerous. 

There  are  four  general  methods  of  lining  up  an  aeroplane: 

1.  By  level  and  plumb  bob,  in  a  factory  or  shed,  with  a  solid 
floor. 

2.  By  transit,  projection  of  angles  and  levels.     (The  most    ac- 
curate method,  and  of  great  convenience  in  a  factory  in  setting  up.) 

3.  By  measurements  of  cross  distances  and  wire  lengths,  com- 
bined with  sighting. 

4.  By  sighting  alone. 

Methods  1  and  2  are  obviously  capable  of  great  accuracy  in  a 
factory,  and  should  be  resorted  to  when  the  aeroplane  is  first  con- 
structed. 


123 

Methods  3  and  4,  may  be  used  anywhere  at  all,  on  the  side  of  a 
hill,  if  necessary,  and  directly  concern  us  here  in  reassembling  a  machine 
for  use  on  the  field. 

Types  of  aeroplanes  differ  widely  among  themselves,  and  actual 
instructions  and  measurements  for  lining  up  aeroplanes  are  required 
to  be  furnished  in  complete  detail  by  the  manufacturer. 

Certain  general  principles  of  importance  are  involved,  however, 
to  which  special  attention  must  be  given. 

All  bolts  on  fittings  should  be  tightened  before  any  trueing  up 
of  wires  is  attempted. 

Turnbuckles,  when  assembling,  should  first  have  the  barrel  taken 
off  and  then  be  started  even  and  turned  up  about  6  or  8  turns  —  enough 
to  give  a  firm  hold. 

In  tightening  up  any  wires,  as  already  indicated,  the  tension  should 
not  exceed  that  required  to  make  the  wire  just  taut  and  free  of  sag  or 
vibration. 

In  placing  bolts  in  fittings,  or  in  spars,  great  care  should  be  taken 
to  see  that  the  threads  are  neither  worn  nor  burred,  and  that  the  bolts 
are  not  forced  in  too  strongly  —  since  they  have  been  made  to  fit  well 
and  are  driven  home  best  by  carefully  directed,  easy  pressure. 

Under  no  circumstances  should  wings  and  their  parts  be  ham- 
mered and  jerked  into  place,  since  if  the  parts  do  not  fit  together  snugly 
and  smoothly  there  will  be  some  extra  strain  somewhere,  when  they 
are  finally  assembled.  In  this  connection  it  must  be  borne  in  mind, 
however,  that  treated  linen  covering  may  tighten  so  much  on  a  wing 
frame  that,  if  left  standing  for  a  long  time,  it  may  twist  and  warp  the 
wing  out  of  shape. 

Assuming  a  biplane  of  the  common  tractor  type,  with  a  small 
center  section  and  wings,  each  of  two  panels,  in  which  the  chassis  and 
body  are  assumed  to  be  in  proper  alignment,  it  may  prove  of  interest 
to  consider  the  assembly  and  trueing  up  by  methods  (3)  and  (4),  out- 
lined above. 

Assembly   and   Alignment   by   Cross   Distances. 

The  several  steps  in  the  assembly  are  indicated  by  referring  to  the 
sketch  on  p.  124.  To  begin  with,  the  center  section  of  wing  over  the 
body,  is  set  over  the  body,  on  the  four  small  struts.  The  first  step  in 
alignment  is  to  make  this  center  section  parallel  to  the  body  and  centered 
over  it.  Since  the  body  is  lined  up,  and  the  section  aff'a',  is  a  parallelo- 
gram, it  follows  that  the  cross  distances,  indicated  as  af,  may  be  made 
equal,  in  order  to  center  up.  When  this  is  done  for  both  front  and  rear 
trusses,  the  center  section  is  bound  to  lie  parallel  to  the  body  axis,  provid- 
ing, of  course,  the  distances  were  measured  between  symmetrical  points. 


124 


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Diagrams  for   Alignment. 

It  then  remains  to  adjust  the  front  and  rear  wires  until  the  section  has 
been  pulled  forward  or  back,  so  that  one  measurement  f"a  agrees  with 
the  similar  one  on  the  other  side  of  the  body  and  with  the  data  on  the 
machine.  But  these  wires  should  not  be  tightened  up  until  the  wings 
are  on,  in  order  to  give  play  for  the  spar  fittings  of  the  wing  section. 
Unless  the  center  section  is  somewhat  near  centered,  however,  difficulty 
will  be  found  in  fitting  the  rest  of  the  wings.  The  next  step  is  to  fit  the 
lower  wings  on  either  side  to  the  body,  and  to  hold  them  up  by  means 
of  their  landing  wires,  fastened  to  the  proper  fittings  at  aa',  but  not 
tightened  up.  The  top  wings  next  to  the  body  are  then  fastened  to  the 
center  section  and  held  in  place  by  hand  until  the  struts  de,  d'e',  are 
inserted,  when  the  landing  wire  ae,  a'e',  will  hold  both  wings  in  place. 
If  the  wings  have  no  dihedral  and  the  fittings  are  symmetrical,  the 
distances  ae  and  bd,  should  be  equal  and  can  readily  be  made  so,  by  tak- 
ing up  the  landing  wire  on  both  sides,  front  and  rear.  This  will  then  give 
the  proper  setting  laterally.  If  a  dihedral  is  employed,  there  will  be 
differences  in  the  measurements,  ae  being  shorter  than  bd,  but  for  proper 
alignment  it  is  merely  necessary  to  have  similar  wires  on  the  other  side, 
the  same  length.  Of  course,  it  is  assumed  that  the  struts  and  the  size 
and  position  of  fittings  on  the  spars  are  unalterably  correct.  The  outer 
sections  may  now  be  put  on  by  the  same  method,  the  lower  one  first,  held 
by  the  landing  wires,  and  then  the  top  one,  supported  on  the  struts, 
and  the  cross  distances  made  equal  similarly  by  taking  up  on  the  landing 
wire.  The  entire  wing  structure  is  now  assembled,  attached  to  the  body, 
which  is  resting  on  the  chassis.  It  is  assumed  that,  laterally,  the  wings 
are  symmetrical  to  the  body  and  properly  transverse  thereto.  This  is 
readily  checked  by  measuring  the  two  distances,  a  h  and  c  d,  as  indicated. 
The  cross  wires  running  from  front  to  rear  between  the  struts  are  next 
adjusted,  just  to  tautness,  and  the  alignment  of  the  struts  as  viewed 
from  the  side  is  checked  by  eye.  Measurements  of  these  cross  distances 
from  top  front  to  lower  rear,  and  lower  front  to  top  rear,  at  the  body, 
are  then  carried  out  to  the  tips,  and  thus  the  angle  of  incidence  is  checked. 
The  "flying  wires"  are  then  all  tightened  up,  just  so  as  not  to  give  the 
landing  wires  more  than  the  strain  of  carrying  the  weight.  Final  meas- 
urements are  then  made  from  the  rear  point  of  the  tail  to  panel  points 


125 

h,  h',  out  at  the  ends  of  the  wings,  in  order  to  determine  if  the  transverse 
wing  axis  is  symmetrical  with  the  longitudinal  body  axis.  The  machine 
is  then  lined  up  correctly  —  providing  that  the  distances  measured  are  all 
taken  to  some  points  on  fittings  or  marks  on  spars  and  struts,  that  are 
absolutely  symmetrical  for  the  two  sides. 

Alignment  by   Sighting. 

The  process  of  assembly,  as  outlined  above,  would  be  the  same. 
After  the  wings  are  attached  to  the  frame,  the  trueing  up  process  proper 
begins.  The  method  consists  merely  in  doing  by  eye  what  was  done  in 
the  previous  example  by  extensive  measurements.  The  first  sight  is 
taken,  from  below  the  body,  up  to  the  center  section,  so  as  to  get  the 
points  a,  a',  over  the  points  b,  b'.  Both  sides,  front  and  rear  are  sighted 
and  the  positions  averaged  up  by  the  wires.  In  assembling,  however, 
no  lining  up  is  done,  until  all  the  wings  are  on,  held  by  the  landing  wires. 
The  observer  then  stands  at  s,  to  one  side,  and  sighting  along  the  top 
plane,  establishes  the  line  across  the  bolt  heads  or  fittings  at  a,  a',  and 
proceeds  first,  to  bring  up  d,  d',  by  means  of  the  wires  a  e,  a'e',  and  then 
g  g',  on  either  side,  are  brought  up  by  taking  up  their  landing  wires  until 
they  are  in  line  with  a,  a',  d,  d',  etc.  In  other  words,  the  transverse  line, 
across  the  top  of  the  center  section,  is  projected  to  either  side.  The  same 
is  done  for  the  rear  spar,  and  then  the  load  wires  for  the  front  spar  only 
are  tightened,  just  enough  so  that  when  the  point  h,  for  example,  is  al- 
ternately raised  and  lowered  no  wires  are  seen  to  slack  or  sag,  —  the 
alignment  held  by  the  landing  wires  being  the  correct  one  to  be  held. 

The  final  and  important  element  in  the  sighting  is  to  establish  the 
correctness  and  uniformity  of  the  angle  of  incidence,  which  is  the  main 
object  of  the  alignment.  To  do  this  best,  the  observer  stands  15  to  20 
feet  in  front  of  the  fuselage,  taking  care  to  center  himself,  by  sighting 
along  the  center  struts,  shaft,  axle  center,  tail  piece,  etc.  The  observer 
then  chooses  a  height,  or  tilts  the  machine,  so  that,  when  sighting  along 
the  top  plane,  he  can  see  just  a  little  of  the  under  side.  This  permits 
him  to  see  a  certain  point  of  the  rear  strut  sockets  showing  against  the 
lower  side  of  the  front  beam.  Then,  by  holding  the  head,  central,  and 
just  high  enough  to  see  these  points,  and  moving  only  the  eyes,  to  right 
and  left,  he  can  note  any  lack  of  alignment  of  the  rear  spar,  parallel  to 
the  front  spar.  This  is  corrected  by  means  of  the  rear  spar  landing  wires, 
if  necessary,  after  which  the  rear  load  or  flying  wires  are  also  tightened. 

The  fore  and  aft  cross  wires  are  now  set,  so  that  when  standing  10 
feet  or  so  from  either  end  of  plane  all  struts  will  lie  in  line  and  parallel 
with  each  other  and  with  the  center  struts.  These  wires  are  then  set  no 
tighter  than  necessary,  for  if  too  tight  they  merely  tend  unduly  to  com- 
press and  buckle  the  ribs. 

A  check  on  the  perpendicularity  of  the  transverse  wing  axis  to  the 
longitudinal  body  axis  is  then  made  by  measurement,  and  if  necessary, 
adjusted  by  the  "drift"  wires  running  from  the  nose  of  the  machine  to 
the  front  intermediate  struts. 


126 


The  tail  pieces  are  then  lined  parallel  to  the  wing  axis,  by  merely 
sighting  and  adjusting  them  until  they  are  parallel.  It  is  well  to  sight 
from  behind  and  below,  so  as  to  get  the  tail  line  just  below  the  front  edge 
of  the  top  plane. 

The  last  wires  to  be  tightened  are  any  auxiliary  wires  from  the  chassis 
to  the  wings. 

This  method,  in  the  hands  of  one  who  has  had  some  experience,  is 
the  quickest,  easiest,  and  accurate  enough  for  field  work. 

A  judicious  combination  of  the  sighting  method  and  the  method  of 
measuring  cross  distances,  gives  the  best  results  in  the  alignment  or 
trueing  up  of  aeroplanes. 

Particular  attention  is  called  to  the  systematic  manner  of  doing 
the  aligning  with  the  landing  wires,  leaving  the  tightening  of  the  "flying" 
wires  to  the  very  last  thing. 

On  the  diagram,  a  note  is  given  relative  to  the  importance  of  loosen- 
ing up  the  proper  wires  when  a  local  adjustment  of  one  panel  point  is 
made,  on  a  machine  already  all  wired  up. 


I    Propeller    Diagram    and    Balancing    stand. 


Propeller   Balance. 

After  the  machine  is  assembled  and  lined  up  the  propeller  may  be 
mounted,  but  before  doing  so  its  balance  should  at  least  be  checked  up. 
A  propeller  "out  of  balance"  is  heavier  on  one  blade  than  on  the  other, 
and  when  run  on  the  engine  will  vibrate.  Any  vibration  of  this  nature 
is,  really,  a  severe  strain  on  the  machine,  and  particularly  on  the  engine. 
A  propeller  may  also  be  troublesome  in  vibrating  if  the  blades  are  warped, 
and  lacking  in  symmetry.  This  may  be  checked  up  by  measurements 
of  offsets  on  the  blade. 

The  accompanying  diagram  shows  a  method  of  propeller  balancing 
that  is  effective,  and  also  shows  the  manner  in  which  the  useful  data  on. 
the  shape,  section  and  angles  of  the  blade  may  be  presented. 


127 

If  the  propeller  is  slightly  out  of  balance,  a  little  more  varnish  on 
the  light  side  is  the  best  way  to  equalize  it. 

Metal  tips  along  the  entering  edge  of  the  tip  of  the  blades  are  a 
great  protection  against  both  water  and  shrubbery,  to  prevent  cracking 
and  splitting  of  the  edge  of  the  blade.  These,  however,  must  be  very 
firmly  attached  and  because  of  the  centrifugal  force  should  be  made  as 
light  as  possible.  For  water  work,  it  is  necessary  to  bore  a  few  small 
holes  in  this  metal  tipping,  in  order  that  the  water,  that  has  soaked  in 
by  impinging  so  hard,  may  be  freely  thrown  off  by  centrifugal  force, 
instead  of  tending  to  work  in  under  and  finally  to  split  open  the  metal 
tipping,  and  for  land  work  such  holes  will  prevent  "dry  rot." 

Attention  should  also  be  given  the  propeller  bolts  to  make  sure  that 
they  are  properly  proportioned  as  to  thread,  that  the  nut  fits  and  shows 
no  sign  of  having  been  forced,  and  that  the  bolts  are  properly  locked 
by  a  wire,  which  is  not  likely  to  be  cut  by  the  nut  of  the  bolt  "backing 

off." 

Details   of   Construction. 

Examination  of  the  details  of  construction,  to  make  sure  of  the 
proper  fitting  of  parts  and  "follow  thru,"  is  most  important,  and  special 
training  in  the  proper  inspection  of  machines  is  next  in  importance 
to  training  in  flying.  No  matter  how  well  built  or  how  reliable  struc- 
tural features  appear  to  be,  there  is  always  the  possibility  of  breakage. 
Just  because  an  aeroplane  has  flown  very  successfully  is  no  excuse  for 
being  any  the  less  careful  in  inspection  of  its  construction. 

It  is  well,  first,  to  go  over  the  entire  machine  and  make  sure  that 
all  the  bolts  are  locked,  and  while  doing  so  the  material  of  the  bolt, 
whether  special  steel  or  "commercial"  iron  bolts,  should  be  examined, 
and  also  the  thread  of  the  bolt,  and  fit  of  the  nut  and  its  locking.  If 
iron  bolts  (stove  bolts)  are  found,  with  deep  threads,  in  places  taking 
any  vital  stress,  they  should  be  replaced. 

Bolts  may  be  locked  in  four  ways : 

1.  By  a  lock  washer,  or  cut  washer,  fitting  under  the  nut  and 
"biting"  into  it  when  the  nut  turns  backwards. 

2.  By  a  pin,  or  lock  wire,  passing  thru  a  hole  drilled  into  the  bolt, 
and  fastened  in  such  a  way  that  vibration  will  not  permit  "backing  off" 
of  the  nut,  to  break  the  locking  wire. 

3.  By  riveting  the  head  of  the  bolts.     This  is  the  most  positive 
lock. 

4.  By   painting  the  bolt  head.     This  is  suitable  only  where   a 
small,  relatively  unimportant  fitting  is  concerned. 

The  practice  of  "spoiling  the  thread"  of  the  bolt  for  locking  is  not  a 
reliable  one. 


128 

Knowing  the  comparative  strengths  of  various  bolts  in  shear  and 
pull,  as  outlined  in  the  table  p.  136,  the  inspection  will  intelligently 
reveal  the  uniformity  of  "safety  factor"  and  "follow  thru." 

After  attending  to  the  bolts,  the  pins  in  the  fittings  and  the  turn- 
buckles  may  be  examined  at  each  panel  point,  one  by  one  —  the  pins  for 
proper  locking,  unless  already  riveted,  and  the  t.  b.'s  for  the  purpose  of 
making  sure  that  enough  threads  are  everywhere  engaged  in  the  barrel 
and  that  the  t.  b.  is,  in  each  case,  locked  so  that  the  safety  wire  will  not 
wear  or  tend  to  break  at  any  point. 

The  general  inspection  of  the  wires,  struts  and  remainder  of  the 
machine  can  then  be  made,  special  attention  being  given  to  the  controls, 
so  as  to  make  sure  that  they  are  connected  up  to  work  properly,  and 

that  all  t.  b.'s  and  pins  are  suitably  locked,  with  no  possibility  of  a  cable 
binding  by  running  off  its  pulley,  or  of  parts  of  the  control  "catching" 
anything. 

To  assist  in  the  detection  of  flaws  in  construction,  improper  propor- 
tioning of  parts  for  a  uniform  strength  and  "follow  thru,"  and  for  gen- 
eral information  on  the  construction  of  aeroplanes,  some  tables  and 
data  are  presented.  It  is  perhaps  necessary  to  state  that  the  strength 
values  are  largely  based  on  tests  and  experiences  of  the  writer  relative 
to  aeroplanes,  and  may  be  taken  as  at  least  a  beginning  of  a  handbook 
for  Aviation,  to  which  new  data  of  value  should  constantly  be  added. 

The  illustrations  of  details  of  construction,  with  examples  of  ap- 
parently reliable  and  unreliable  features,  should  receive  particularly 
close  attention  from  military  aviators.  The  small  variety  of  details 
shown  must,  of  course,  be  taken  as  serving  merely  as  examples,  since 
no  attempt  has  been  made  to  present  all  the  structural  features  that 
might  be  found  on  a  various  assortment  of  types  of  aeroplanes. 

In  order  to  avoid  the  inconveniences  of  cross  reference,  notes 
relative  to  the  various  features  have  been  incorporated  on  the  illus- 
trations themselves,  and  should  be  read  and  digested  as  carefully  as  any 
emphasized  text. 

Steel. 

Steel  is  obtained  from  iron  by  many  processes,  differing  in  ore  treat- 
ment, expense,  etc.,  the  most  extensive  ones  being  Bessemer,  open- 
hearth  and  crucible.  All  refer  to  the  original  method  of  obtaining 
the  steel,  and  have  little  bearing  on  the  quality  of  the  steel,  excepting 
in  the  amount  of  carbon,  alloy,  etc.,  in  it.  There  are  many  instances, 
however,  of  Bessemer  process  steel  proving  less  reliable  than  the  others. 
The  crucible  process  is  used  to  obtain  the  most  uniform  tool  steels. 

The  percentage  of  carbon  in  steel  largely  determines  its  hardness, 
strength  and  ductility,  and  ranges  from  .05  %  to  .25  %.  The  higher  the 
carbon,  the  harder,  more  tenacious  and  less  ductile  is  the  steel.  The 


129 

lower  the  phosphorus  or  sulphur,  the  less  likely  is  the  steel  to  develop 
flaws  and  cracks. 

The  word  "temper"  is  used  by  manufacturers  to  represent  the 
amount  of  carbon  in  steel.  Thus,  a  "high  temper"  steel  is  a  "higher 
carbon"  steel,  and  therefore  hard,  tenacious,  but  brittle.  Steels  may  be 
"tempered,"  after  manufacturing  by  applying  various  degrees  of  hard- 
ening and  softening  —  that  is,  most  uniform  steels  can  be  made  as  hard 
and  tenacious,  or  as  ductile  and  soft  as  desired. 

"Hardening"  is  done  by  heating  the  steel  —  with  particular  attention 
to  uniform  heating  of  the  metal  —  and  then  quickly  immersing  in  brine, 
oil  or  water;  the  amount  or  nature  of  this  quick  uniform  cooling,  or  of 
the  heat  to  which  the  steel  was  brought,  being  determined  by  the  kind 
of  hardening  desired  (all  of  which  requires  personal  skill  and  experience). 

"Softening"  of  steel  is  designed  to  make  its  texture  more  uniform, 
easier  to  manipulate,  and  less  brittle.  This  process  is  termed  "anneal- 
ing," and  consists  merely  in  heating  steel  up  to  a  desired  temperature 
and  then  letting  it  cool  very  slowly,  the  slower  the  cooling  the  softer  the 
steel.  As  in  any  other  treatment  of  steel,  uniformity  of  heating  or  cool- 
ing is  of  the  utmost  importance.  Practically  all  high  grades  of  steel 
come  from  the  mills  in  annealed  condition,  but  if  not,  and  if  it  is  de- 
sired to  bend  the  steel  sharply,  great  care  must  be  exercised  in  heat- 
ing it  in  a  forge  for  annealing  to  make  sure  that  the  steel  is  uniformly 
heated,  otherwise  its  grain  and  texture  will  be  uneven  and  weakened. 

In  this  connection,  it  is  important  to  point  out  that  steel  has  as 
marked  a  "grain"  as  wood,  only  not  as  easy  to  see.  Steel  is  always  weak- 
est across  the  grain. 

Alloy  steels,  by  various  heat  treatments,  can  be  made  to  give  various 
strengths,  but  increased  hardness  or  elastic  limit  is  almost  always 
obtained  at  the  expense  of  ductility.  In  the  annealed  condition,  which, 
because  of  the  reduction  in  brittleness,  is  desirable  for  aeroplane  work, 
steels  do  not  show  much  variation.  A  table  is  -given  of  the  strengths 
of  various  grades  of  alloy  steels,  and  the  elongation  or  per  cent  that  any 
length  will  stretch  before  breakage  is  given,  and 'is  an  indication  of  the 
ductility. 

In  aeroplane  work,  it  is  essential  to  have  the  maximum  of  reliability, 
and  since  local  thoughtless  heating  may  have  robbed  a  "special"  steel 
of  its  special  qualities,  it  is  the  best  practice  to  proportion  all  parts  for  a 
ductile,  easily  bent,  mild  carbon  steel,  with  the  strength  given  in  the 
table.  Then,  if  any  advantageous  alloy  like  Vanadium  steel  is  used,  its 
greater  resistance  to  fatigue  is  an  added  and  much  needed  safety  factor. 

The  commercial  names  of  "tool  steel,"  or  "drill  rod"  (bars  of  tool 
steel),  refer  to  a  specially  uniform  and  reliable  grade  of  rather  pure  steel, 
particularly  adapted  to  being  heat  treated,  tempered  and  hardened  for 


1,  2,  3,  4.  Various  single  and  double  pulley  arrangements  for  control  cables, 
—5.  The  Curtiss  double  U  bolt  fitting. —6.  The  Burgess  clip  fitting. —7.  The 
Curtiss  single  U  bolt  fitting.  — 8.  Signal  Corps,  pin  and  plate  fitting.  — 9.  The  steel 
block  and  eye  head  strut  bolt  fitting  used  on  German  aeroplanes.  — 10.  The  Wright 
hook  fitting.  — 11-12.  Hinge  details. 


131 


1.  Control  with  cables  and  pulleys  on  ball  bearings.  —2.  Same  with  friction  leads. 
—3.  Detail  of  rubber  shock  absorber  bridge.  —4.  Steel  Spring  chassis,  with  central 
skid.  —5.  Softer  rubber  chassis  with  no  skid.  Both  of  them  are  typical  chassis  for 
exactly  the  same  work.  — 6.  Fuselage  details.  — 7.  Details  of  wing  frames,  ferrules 
and  lumber. 


132 

special  tool  purposes.     Tool  steel  and  drill  rod,  in  annealed  condition, 
are  good,  mild  steels  for  bolts,  pins,  etc. 

Bolts,  pins,  turnbuckles,  and  particularly  wires  and  cables,  may  often 
be  of  heat-treated  special  chrome  nickel  or  vanadium  steel,  and  care  must 
be  taken  not  to  heat  unequally  any  of  these  parts,  and  thus  reduce 
the  added  safety  factor  they  furnish.  This  is  particularly  important 
in  the  case  of  steel  wires  and  cables,  in  which  the  material  and  method 
of  drawing  of  the  wire  have  been  designed  particularly  to  give  a  high 
tension  strength,  which  any  local  heating,  for  the  purposes  of  bending 
or  attachment,  may  very  seriously  weaken.  For  example,  a  tension 
brace  of  a  particularly  fine  grade  of  piano  wire,  received  undamaged 
from  the  manufacturer  and  properly  put  into  place,  may  be  relied  upon 
to  give  its  average  tested  breaking  strength.  But  let  this  same  wire 
come  into  long  contact  with  a  torch  flame,  being  used  to  bend,  solder  or 
braze  some  fitting,  and  it  may  well  have  been  reduced  in  strength  to  one- 
third  of  what  it  is  supposed  to  be. 

Cold  rolled  steel  (abbreviated  c.  r.  s.),  which  is  used  so  largely  in 
aeroplane  work,  in  fittings,  ferrules,  clips,  etc.,  is  steel  that  has  been 
rolled  out  to  the  sheet  or  bar  in  question,  but  in  doing  so  the  grain  of  the 
steel  becomes  more  marked.  This  steel  is  harder  and  more  tenacious 
than  mild  annealed  steel,  but  works  very  easily  and  has  splendid  wearing 
qualities.  Bends  in  c.  r.  s.,  however,  should  not  be  made  too  sharp, 
and  when  plate  more  than  1-8"  thick  is  used,  care  should  be  taken  to 
anneal  before  bending,  or  else  to  bend  slowly  in  a  vise  in  which  the  jaws 
are  protected  by  thick  copper  pads,  to  avoid  nicking  the  plate. 

Other   Metals. 

The  table  on  p.  136  gives  the  strengths  and  weights  of  other  metals, 
but  they  are  rarely  used  in  the  parts  of  an  aeroplane  carrying  the  main 
stresses,  excepting  the  bronze  barrels  of  turnbuckles. 

Aluminum  should  never  be  used  in  any  important  fitting,  and  its 
alloys,  though  at  times  exhibiting  remarkable  characteristics,  are  almost 
as  unreliable  as  aluminum  itself.  Many  of  them,  however,  are  ad- 
vantageously of  use  in  castings,  sheet  metal  coverings,  etc.,  requiring 
a  metallic  construction,  but  carrying  no  great  stress.  Duralumin  has 
very  nearly  the  strength  of  mild  steel,  in  spots,  and  is  somewhat  more 
weather  and  water-resisting  than  any  of  them.  Aluminum  sheeting 
should  never  be  used  on  coverings  in  sheeting  of  less  than  1-1 6th  inch 
thick,  as  it  eventually  flakes  and  cracks. 

Tin  and  copper  are  used  for  the  ferrules  of  wire  joints  and  for 
tankage. 

"Monel"  metal,  an  alloy  of  about  the  same  qualities  as  mild  steel, 
is  extensively  used  on  metal  fittings  where  particular  rust  resisting  quali- 
ties are  desired. 


133 
Crystallization   and   Fatigue. 

The  wearing  down  of  the  resisting  qualities  of  a  material  by  constant 
vibration  and  jar,  is  a  familiar  phenomenon  met  in  practical  engineering 
of  all  kinds  —  so  much  so,  that  a  certain  "life"  is  assigned  to  metal  parts, 
after  which  their  strength  is  considered  unreliable.  This  should  be 
followed  in  relation  to  aeroplane  metal  fittings,  but  a  great  error  is  made 
in  attributing  so  much  danger  to  "crystallization"  in  the  failure  of  parts, 
since  the  vibrations  on  aeroplanes  are  neither  sharp  nor  excessive. 

"Fatigue,"  is  the  destruction  of  the  resisting  qualities  of  a  material 
by  repeated  strains  of  bending  or  twisting,  exceeding  what  the  "springi- 
ness" of  the  material  will  stand,  as  illustrated  by  the  ease  with  which  a 
wire  can  be  broken  by  repeated  twisting  or  a  steel  plate  by  repeated 
bending.  It  is  of  the  utmost  importance,  then,  to  make  sure  that  the 
structural  details  are  not  such  as  to  permit  the  pull  or  flexing  of  a  part 
to  result  in  bending  or  twisting  strains  on  details  not  suitably  made  for 
them.  Attention  to  some  examples  of  this  is  given  in  the  illustrations 
of  structural  details. 

In  the  construction  of  military  aeroplanes  it  is  desirable  to  eliminate 
brazed  and  welded  fittings  as  much  as  possible,  not  only  because  of  the 
added  difficulty  of  replacement,  but  because  a  welded  joint  does  not 
always  reveal  a  possible  flaw  to  the  naked  eye,  and,  though  apparently 
satisfactory,  might  actually  prove  dangerously  inadequate  for  its  stress. 
Practically  all  aeroplane  fittings  may  be  made  of  simple  and  effective 
steel  plate  clips,  as  light  and  as  strong  as  more  "refined"  and  elaborate 
arrangements- — refined  only  in  that  they  are  harder  to  make,  replace,  and 
pass  on. 


CABLES 


WIRES 


HARD  WWE. 


THf  iTRAHDS  AKt 
!>  ffiLfP  WITH  SCLDfR  TO 
PWC. 


L[/VC,TH  or  fcxrrvLE 
-?%." 


8t  SCMffft 
lH»f*  BINT 


Cable   and   Solid   Wire   Ends. 


134 
Aeroplane  Woods 

For  use  in  the  construction  of  aeroplanes  wood  has  peculiar  virtues, 
one  of  the  best  of  which  is  the  ease  with  which  flaws  can  be  detected. 
In  this  connection,  it  is  a  great  mistake  to  paint  wooden  parts  on  aero- 
planes, since  varnish,  or  "dope,"  will  give  as  good  preservation  and  yet 
bring  out  clearly  in  evidence  any  defective  features. 

Among  the  woods  used  in  aeroplane  work  attention  may  profitably 
be  given  to  Spruce,  Ash,  Maple,  Hard  Pine,  Walnut,  Mahogany,  Cedar 
and  Hickory,  strengths  and  weights  of  which  are  given  in  the  table. 

Spruce,  of  clear  silver  grain,  straight,  smooth  and  free  of  knotholes 
or  sap  pockets,  is  the  lightest,  strongest  and  most  generally  satisfactory 
material  for  aeroplane  construction  available.  It  must  be  properly 
ferruled,  where  fittings  are  attached,  however,  to  prevent  splitting. 
As  a  material  for  spars,  ribs,  struts,  etc.,  it  gives  a  splendid  combination 
of  flexibility,  lightness  and  strength. 

Ash  is  springy,  strong  in  tension,  hard,  and  very  tough.  Its  weight, 
however,  is  considerably  greater  than  spruce,  which,  when  properly 
ferruled,  can  for  the  same  weight  be  made  stronger  than  any  other 
wood. 

Maple  has  excellent  qualities,  in  strength  and  reliability,  for  very 
small  wood  details  requiring  unusual  resisting  powers — like  the  blocks 
connecting  rib  pieces  across  a  spar. 

Hard  Pine  is  a  tough,  uniform  wood,  particularly  applicable  to 
members  like  the  "longerons"  of  fuselages  (longitudinal  members). 

Walnut  and  Mahogany  are  used  extensively  on  propellers,  their 
uniformity  in  finishing  and  strength  giving  excellent  results  for  this 
purpose. 

Cedar  is  often  used  as  planking  of  hulls,  or  fuselage  covering,  is 
readily  obtained  in  the  boards,  and  quite  uniform  and  easily  worked. 

In  this  connection,  fuselages,  particularly  "monocoques,"  are  some- 
times made  of  veneers,  or  glued  layers  of  wood,  with  tr  e  grains  crossing 
for  added  strength.  Tulip  wood,  bass  wood,  cedar,  alder  and  mahogany, 
are  used  for  veneer  covering  work.  There  are  innumerable  trade  makes 
of  "veneers,"  some  of  them  very  satisfactory  in  aeroplane  work. 

Hickory,  which  is  tough  and  springy,  and  with  a  hard  surface,  is  a 
favorite  material  for  skids,  control  levers,  etc. 

For  the  preservation  of  wood  several  coats  of  spar  varnish,  or  of 
aeroplane  dope,  should  be  used,  after  an  original  "filler"  of  oil  or  shellac. 

Laminations  in  wooden  members  are  designed  to  make  splitting  of 
the  member  more  difficult  by  having  different  layers  of  wood  with  the 
grain  running  in  opposite  directions,  glued  firmly  together.  Weather- 
ing, however,  is  apt  to  affect  the  glue  and  open  the  laminations,  and  it  is 
good  practice  to  wrap  the  members  with  linen  or  paper,  or  to  freshen  up 
the  paint  or  varnish  from  time  to  time. 


135 

The  wrapping  of  wooden  members  with  linen,  may  be  made  to 
increase  the  strength  against  splitting,  if  the  linen  is  wound  very  tight 
and  treated  with  "dope"  or  glue  in  such  a  way  that  it  will  forcibly 
tighten  up.  The  "dope"  should  be  renewed  from  time  to  time. 

Due  to  the  necessity  of  having  a  certain  least  width  to  a  strut,  so 
that  the  ratio  of  the  length  of  a  strut  1  to  its  least  width  r  will  not  exceed 
by  too  great  a  margin,  the  1/r  of  45,  that  engineering  practice  prescribes 
as  a  limit,  wooden  struts,  particularly  of  spruce,  are  better  than  steel  or 
any  other  material, — for  the  saving  in  width  and  therefore  head  resist- 
ance of  a  stronger  material,  would  sacrifice  strength  against  bending. 

Experience  in  being  able  to  pick  out  good  lumber  and  detect  flaws, 
is  of  great  benefit,  and  should  in  a  measure  be  acquired  by  any  aviator 
who  is  interested  enough  in  his  machine  to  desire  assurance  as  to  its 
strength. 

Wing  Covering. 

The  general  practice  in  wing  construction  is  to  cover  the  rib  and  spar 
framework  with  an  air-tight  cloth,  giving  a  smooth  finish  to  the  surface 
and  some  degree  of  resistance  to  deterioration  by  exposure. 

Rubbered  fabrics  were  used  for  several  years,  but  it  was  necessary 
to  tighten  them  by  hand  in  stretching  on  the  frame,  and  the  cloth  would 
sag  in  dry,  sunny  weather,  and  tighten  in  damp  weather. 

An  improvement  in  covering  was  made  by  the  adoption  of  fine, 
unbleached  linen,  which  is  stretched  rather  loosely  on  the  wing  frame, 
and  is  then  treated  with  "dope." 

"Dopes"  are  of  several  kinds,  but  they  are  almost  all  cellulose  or 
"collodion"  compounds,  some  soluble  in  ether  and  some  in  aceton. 
"Cellon,"  "Novavia,"  "Emaillite,"  "Cavaro,"  "Titanine,"  are  but  a 
few  of  the  trade  names,  all  with  some  particular  virtue — some  fireproof, 
others  lacking  in  bothersome  chemical  odors,  but  all  designed  to  accom- 
plish the  same  purpose,  i.  e.,  to  tighten  up  the  linen  on  the  frame,  and 
after  a  few  coats,  applied  with  a  brush,  to  give  to  the  surface  a  smooth, 
weather-resisting  finish. 

Skill  in  applying  dopes  and  various  "formulae"  for  the  processes, 
give  varying  degrees  of  finish,  but  in  general  four  or  five  coats  of  a 
tightening  solution,  followed  by  three  coats  of  a  thicker  finishing  solution, 
will  give  a  good  finish.  It  is  customary  to  varnish  this  covering  with  spar 
varnish,  after  the  dope  has  set,  but,  in  view  of  the  difficulty  of  patching 
and  "re-doping"  over  the  varnish,  the  advisability  of  this  practice  is 
questionable.  To  clean  most  doped  fabrics,  some  soap  and  water  will 
be  found  better  than  anything  else. 

The  linen  fabric  used  for  this  covering  is  woven  in  the  customary 
way  with  "warp,"  the  yarn  running  lengthwise,  and  "weft,"  the  yarn 
running  across  the  cloth. 


136 


Good  aeroplane  linen  should  test  to  a  tension  of  at  least  50  Ibs.  for 
1-inch  width  strip  of  cloth  undoped,  and  should  be  difficult  to  tear  and 
rip.  When  doped  it  should  show  a  strength  of  at  least  70  Ibs.  per  inch. 

Cloth  with  a  fine  thread  is  not  quite  as  strong  as  cloth  with  a  coarser 
thread,  but  the  latter  absorbs  very  much  more  "dope"  for  a  good  finish. 

Aeroplane  linen,  doped  to  a  good  finish,  weighs  approximately 
0.10  Ibs.  per  sq.  ft.  of  surface,  inclusive  of  tape  or  batten  rib-covering 
and  varnish,  for  both  top  and  bottom  faces  of  a  surface  taken  together. 

WEIGHTS  AND    STRENGTHS   OF   METALS 


Weights       Elastic  Limit                      Ultimate 
per  cu.  in.        Tension        Tension  Compression     Shear 
Steel  c.  r.  s  283           35,000          50,000         50,000        40,000 
Steel,  piano  wire  280,000        300,000          
Aluminum  096           10,000           15,000         12,000         10,000 
Duralumin  103           29,000           45,000         50,000         40,000 
Tin  265             3,000            3,500           6,000          4,000 
Brass  310          20,000           25,000        30,000        30,000 
Mn.  Bronze  319           50,000          50,000        80,000         70,000 
Copper  320           12,000          20,000        30,000         20,000 

Modulus  of 
Elasticity 
29,000,000 
30,000,000 
11,000,000 

4,000,000 
9,000,000 
14,000,000 
16,000.000 

All  strengths  are  in  Ibs.  per  sq.  inch  and  averages. 
WEIGHTS   OF   SHEET   METAL 


B&S 

Thickness 

Steel 

Gauge 

Inches        1 

bs.  per  sq. 

2 

.258 

10.5 

5 

.182 

7.4 

8 

.128 

5.24 

10 

.102 

4.16 

12 

.081 

3.30 

14 

.064 

2.62 

16 

.051 

2.07 

18 

.040 

1.64 

20 

.032 

1.31 

22 

.025 

1.03 

24 

.020 

0.82 

Aluminum      Brass  or  Copper 
Ibs.  per  sq.  ft.     Ibs.  per  sq.  ft. 


3.59 
2.53 
1.79 
1.42 
1.13 
0.89 
0.71 
0.56 
0.45 
0.35 


per  sq. 
11.6 
8.2 
5.8 
4.6 
3.65 
2.90 
2.3 
1.83 
1.45 
1.14 
0.91 


Tension  of  c.  r.  s.  steel  plate  in  Ibs.  per  inch  width  =  Thickness  X  50,000. 
Bearing  strength  of  wire  in  plate  =  diam.  wire  X  thickness  plate  X  50,000. 


STRENGTHS    OF   VARIOUS 

Kind  of  Steel 
Softest  Low  Carbon  Steel 

GRADES    OF 

Elastic 
Limit 
25,000 

STEEL 

Ultimate 
Strength 
45,000 

Elongation 

28% 

Commercial  Mild  Carbon  Steel,  annealed  
Chrome  Nickel  Steel,  annealed  
Type  "D"  Vanadium  Steel,  annealed  
Chrome  Nickel  Steel,  -tempered  
Type  "D"  Vanadium  Steel,  tempered  

35,000 
55,000 
67,000 
134,000 
195,000 

55^000 
80,000 
100,000 
150,000 
210,000 

£*O   /Q 

20% 
25% 
26% 
15% 
10% 

STEEL  BOLTS 


Diam. 
Inches 

1/8 

3/16 

1/4 

1/4 

5/16 

5/16 

3/8 

1/2 

5/8 

3/4 


No.  of  Threads 

to  the  Inch 
40  U.  S.  St. 
32  U.  S.  St. 
20  U.  S.  St. 
28  A.  L.  A.  M. 
18  U.  S.  St. 
24  A.  L.  A.  M. 
24  A.  L.  A.  M. 
20  A.  L.  A.  M. 
18  A.  L.  A.  M. 
16  A.  L.  A.  M. 
14  A.  L.  A.  M. 


Single 

Diam.  at 
Root 
.092 

Tension       Shearing 
@  50,000  @  40,000 
320                256 

.147 

880               704 

.185 

1,350            1,080 

.205 

1,650            1,320 

.253 

2,500            2,000 

.271 

2,865            2,292 

.321 

4,050            3,240 

.435 

7,500           6,000 

.553 

11,900           9,520 

.669 

17,650          14,120 

.907 

32,500         26,000 

137 


STRENGTH    OF   MILD    STEEL   RIVETS   AND   PINS 


Diam. 
Inches 

Lbs.  Strength 
Double  Shear 

Diam.          Lbs.  Strength 
Inches          Double  Shear 

1/8 

1100 

3/8 

9,500 

3/16 

2400 

1/2 

17,600 

1/4 

4400 

3/4 

39,000 

5/16 

6900 

1 

70,000 

For  single  shear  take  1/2  loads  given. 

CABLES 

Breaking 

Diameter                   No.  of 

Wt.  Lbs. 

Strength  in 

Inches                       Wires 

per  100  ft. 

Pounds. 

1/32  R  

7 

0.35 

200 

1/16  R  

19 

0.96 

500 

1/16  R 

flexible 

400 

3/32  R  

19 

2^6 

899 

.091  MS.  .  .  . 

1000 

7/64  R  

!!.!!!!!!       19 

2.8 

1400 

.118  MS.  .  .  . 

2100 

1/8  R  

'.  .'  .'  .'  .'  '.  '.  '.            19 

'3.'6 

2300 

.138  MS  

3000 

5/32  R  

19 

'S.5 

3000 

3/16  R  

19 

7.2 

3600 

.158  MS.  .  .  . 

4000 

.209  MS.  .  . 

6000 

1/4  R  

".'.'.'.'.'.'.'.           19 

13.8 

8300 

R=  " 

Roebling" 

MS  =  "Morane  Saulnier" 

SOLID   WIRES 

Breaking 

Diameter 

Gauge 

Wt.  Ibs. 

Strength  in 

Inches 

or  descr.             p 

er  100  ft. 

Pounds. 

.032 

20  R 

.264 

225 

.040 

19  R 

.436 

340 

.051 

16  R 

.718 

540 

.055 

ASW, 

.78 

530 

.064 

14  R 

1.13 

830 

.065 

ASW 

1.21 

680 

.080 

ASW 

1.80 

1000 

.081 

12  R 

1.82 

1300 

.090 

ASW 

2.26 

1300 

.100 

ASW 

2.90 

1500 

.102 

10  R 

2.91 

2000 

.130 

ASW  Van. 

4.50 

3000 

.250 

ASW  Van. 

16.00 

5000 

R  and 

No  =  gauge 

Roebling.     ASW  =  American  Steel  and  Wire  Co. 

STEEL   TUBE 

TABLE 

Outside 
Diam. 

Area  of          Wt.  per 
Section              foot 

Moment  of 

Rad.  of     Lbs.  Tension 

Inches 

1/2 

1/2 

3/4 

Thickness 
20  ga. 
1/16" 
18  ga. 

sq.  in.             length 
.051                  .17 
.  086                  .  30 
.108                 .37 

Inertia  I 
.0014 
.0021 
.0067 

Gyr.  r. 
.165 
.156 

.248 

@  30,000 
1,530 
2,580 
3,240 

3/4 

1/16" 

.135                 .46 

.0080 

.244 

4,050 

20  ga. 
1/16" 

1/8" 

.106                  .36 
.  184                  .  63 
.344               1.17 

.0124 
.0203 
.0336 

.341 
.332 
.313 

3,180 
5,520 
10,320 

1/4 
11/4 
11/4 
11/2 
11/2 
11/2 

21/2 
3 

20  ga. 
1/16" 
1/8" 
1/16" 
1/8" 
3/16" 
3/16" 
1/4" 
1/4" 

.  134                  .  45 
.  233                 .  79 
.442               1.50 
.  282                  .  96 
.540               1  .  84 
.773               2.63 
1.07                 3.63 
1.77                 6.01 
2.16                 7.34 

.  0247 
.0412 
.0708 
.0730 
.1287 
.1699 
.4431 
1.132 
2.059 

.430 
.420 
.400 
.509 
.488 
.469 
.644 
.800 
.976 

4,020 
6,990 
13,260 
8,460 
16,200 
23,190 
32,100 
53,100 
64,800 

138 
STANDARD    GAUGES 

No.  of  Gauge    Birmingham    Brown  &  Sharp  United  States 


.380 
.238 
.165 
.134 
.109 
.083 
.065 
.049 
.035 
.028 
.022 


.36480 
.20431 
.12849 
.10189 
.08081 
.06408 
.05082 
.04030 
.03196 
.02535 
.02010 


.34375 
.23437 
.17187 
.14062 
.10937 
.07812 
.06250 
.05000 
.03750 
.03125 
.02500 


Diameter  of 
Amer.  Steel 
&  Wire  Go's 

Gauge 

.3310 

.2253 

.1620 

.1350 

.1055 

.0800 

.0625 

.0475 

.0348 

.0286 

.0230 


^,*±                                              .  \J££,                                         .  \J£*\JL\J  .  \J^^J\J\J 

Birmingham,  used  for  steel  tubes;  B  and  S  for  sheet  metals. 

TURNBUCKLE  TABLE 

Length  of    Length  of   Last  diam.       No.  of  Strength 

Name                   Barrel            ends  of  ends        threads  in  Ibs. 

Burgess 3"               1^"  -2"                 32  3370 

Burgess 3"               1 Y2"  .  175"             32  2470 

3"               iy2"  .15"              32  1700 

5"              21A"  -23"              26  3457 

4^"           2J4"  .20"               26  2492 

3"               1W  .15"              34  1442 


National . 
National . 
National . 


WEIGHT   AND    STRENGTH   OF   WOODS 


Weight  in  Tension  Extreme 

Kind  of  Wood  Lbs.  per  cu.  ft.  Strength  Fibre  Stress 

Ash 50  14,000  6500 

Bamboo    22  6,000  1000 

Cedar 28  5,000  3000 

Hickory 48  13,000  7000 

Hard  Pine 45  12,000  6000 

Mahogany 51  11,000  7000 

Maple  46  10,000  8000 

Oak 52  10,000  6000 

Spruce 32  10,000  5600 

Walnut 42  9,000  5000 

All  strengths  in  Ibs.  per  sq.  in. 

AREAS  AND   VOLUMES 

Triangle.  — Area  equals  one-half  the  product  of  the  base  and  the  altitude. 
Parallelogram. — Area  equals  the  product  of  the  base  and  the  altitude. 
Irregular  figure  bounded  by  straight  lines.  — Divide  the  figure  in  triangles,  and  find 
the  area  of  each  triangle  separately.     The  sum  of  the  areas  of  all  the  triangles  equals 
the  area  of  the  figure. 

Circle. — Circumference  equals  diameter  multiplied  by  3.1416. 
Circle. — Area  equals  diameter  squared,  multiplied  by  0.7854. 

Circular  arc.  — Length  equals  the  circumference  of  the  circle,  multiplied  by  the  num- 
ber of  degrees  in  the  arc,  divided  by  360. 

(Useful  for  tanks,  partly  filled.) 

Circular  sector.  — Area  equals  the  area  of  the  whole  circle  multiplied  by  the  quotient 
of  the  number  of  degrees  in  the  arc  of  the  sector  divided  by  360. 

Circular  segment.  —Area  equals  area  of  circular  sector  formed  by  drawing  radii  from 
the  center  of  the  circle  to  the  extremities  of  the  arc  of  the  segment,  minus  area  of  tri- 
angle formed  by  the  radii  and  the  chord  of  the  arc  of  the  segment. 
Prism. — Volume  equals  the  area  of  the  base  multiplied  by  the  altitude. 
Cylinder. — Volume  equals  the  area  of  the  base  circle  times  the  altitude. 
Pyramid  or  Cone. — Volume  equals  the  area  of  the  base  times  one-third  the  altitude. 
METRIC   CONVERSION  TABLES 


kilometer  =  0.6214  mile 
meter  =  3.2808  feet 
centimeter  =  0.3937  inch 

sq.  meter  =  10.764  sq.  feet 
sq.  centimeter  =  0.155  sq.  inch 

cub.  meter  =  35.314  cub.  feet 
liter  =  0.0353  cubic  foot 

1  kilogram  =  2.2046  pounds 


mile 
foot 
inch 


1.609  kilometer 
0.3048  meter 
2.54  centimeters 


sq.  foot  =  0.0929  sq.  meter 

sq.  inch  =  6.452  sq.  centimeters 

cub.  foot  =  28.317  liters 
U.  S.  gallon  =  3.785  liters 

1  pound  =  0.4536  kilogram 


CHAPTER    XI 
MARINE  AEROPLANES. 


Hydro-aeroplanes  and  aeroboats  involve  all  the  features  of  aero- 
planes that  we  have  considered,  and  in  flight,  whether  land  born  01 
water  born,  no  distinctions  can  be  drawn.  But  in  the  replacement  oi 
landing  wheels  by  watertight  pontoons  for  flotation,  there'  is  intro- 
duced an  important  feature  worthy  of  special  attention. 

Because  of  the  continuous  and  broad  expanse  for  alighting,  and 
the  generally  smoother  air  conditions,  large  water  courses  offer  par- 
ticularly practical  inducements  for  flying,  whether  it  be  for  the  pur- 
poses of  coast  defence  and  naval  operations,  or  for  travel  and  sport. 
And  for  preliminary  instruction  in  flying  there  are  many  who  hold 
— and  justifiably — that  flying  should  first  be  taught  over  water,  be- 
cause of  its  greater  safety,  more  uniform  conditions,  and  continuous 
facilities  for  practice  in  alighting. 

The  general  care  and  maintenance  of  aeroboat  hulls,  or  pontoons, 
differs  in  no  way  from  that  of  high-class  boats,  excepting  that  in  be- 
ing hauled  out  and  in,  with  more  or  less  abuse,  the  light  structure  neces- 
sary is  apt  to  suffer  rather  severe  wear  and  tear. 

The  necessity  of  strongly  braced  construction,  the  best  of  lapped 
and  copper-rivetted  planking,  the  elimination  of  metals  liable  to  rust, 
the  use  of  the  proper  wood  and  its  protection,  so  as  to  avoid  water 
soaking,  protective  keels  and  coating,  all  with  a  minimum  of  weight, 
are  but  applications  of  good  motorboat  practice. 

In  the  form,  functioning  and  adaptability,  of  pontoons  or  hulls  to 
the  aeroplane,  however,  there  is  found  a  specialty  about  which  more 
than  one  entire  textbook  could  profitably  be  written. 

To  assist  in  the  solution  of  difficulties,  in  the  proper  application 
of  pontoons  or  hulls  to  aeroplanes,  a  few  brief  notes  are  presented  here, 
so  that  the  military  or  naval  aviator  may  understand  the  mechanics 
of  water-flying  machines,  sufficiently  to  detect  difficulties  in  balance 
or  "planing,"  and  be  able  to  judge  of  the  suitability  of  various  units 
of  flotation  for  any  particular  machine. 

Air  Resistance. 

Attention  should  be  given  to  having  as  little  disturbance  as  pos- 
sible to  flying  characteristics,  by  the  addition  of  pontoons.  Of  ne- 
cessity, the  floating  members  must  be  low,  and  being  bulky,  more  or 
less  additional  air  resistance  is  introduced.  The  addition  of  this  weight, 
so  low,  appreciably  lowers  the  center  of  gravity. 


140 

Pontoons  have,  generally,  a  considerable  expanse  of  side  surface, 
which  by  being  low  and  at  the  front,  brings  the  directional  center  of  a 
surface  forward,  and  also  introduces  large  fin  effect  below  the  c.  g.,  a 
condition  ordinarily  giving  serious  lateral  instability.  Both  of  these 
features  must  be  cared  for,  preferably  by  adding  a  fin  at  the  rear  and 
giving  a  slight  extra  dihedral  to  the  wings,  or  by  rebalancing  the  ma- 
chine, unless  the  design  was  originally  made  for  water  flying.  Refer- 
ence is  made  to  Chap.  XII,  on  the  significance  of  these  features. 

The  difference  in  resistance  of  pontoons  and  wheels  is  not  nearly 
as  great  as  commonly  supposed,  excepting  at  cabre  attitudes  or  large 
angles  of  yaw.  Some  values  of  K  are  given  on  p.  142  for  several  dif- 
ferent pontoons. 

When  the  fuselage  and  hull  are  combined,*  as  done  in  the  aero- 
boats  or  flying  boats,  efficiency  in  flying  may  actually  be  gained  by 
the  elimination  of  the  resistance  corresponding  to  the  chassis.  Al- 
though the  seaworthiness  of  this  type  is  not  necessarily  greater  or  less 
than  other  types,  the  compactness  of  design  and  gain  in  efficiency 
that  may  be  obtained  by  placing  the  crew,  motor,  etc.,  in  the  hull,  — 
which  of  itself  has  the  proper  strength  and  form  to  serve  as  the  fusel- 
age —  is  considerable,  and  the  entire  craft  becomes  more  boatlike  in 
design,  passing  from  the  "aeroplane  with  floats,"  to  the  "boat  with 
wings." 

Flotation. 

In  order  to  support  the  weight  of  the  machine  on  the  water,  the 
pontoons  or  hull  must  displace  1  cubic  foot  for  every  62  to  64  Ibs.  of 
weight.  The  number  of  cubic  feet  necessary  for  the  total  weight  of 
the  aeroplane,  loaded  will  then  represent  the  volume  of  the  pontoons 
or  hull  "under  water."  The  center  of  flotation  (merely  the  center  of 
this  volume)  will  be  under  the  center  of  gravity. 

The  total  available  amount  of  flotation,  for  any  kind  of  prac- 
tical use,  should  be,  at  the  very  least,  two  and  one-half  times  as  much 
as  this,  and  the  subdivision  of  the  pontoons  or  hull  into  water-tight 
compartments,  is  as  necessary  for  reasons  of  safety  in  flotation  as  it 
is  to  prevent  any  water  that  has  leaked  in,  from  acting  as  a  shifting 
ballast  to  the  detriment  of  the  flying  qualities. 

The  distribution  of  the  flotation  used  and  the  extra  flotation  pro- 
vided must  be  such  that  there  is : 

1.  Ample  flotation  at  the  rear  of  the  c.  g.,  in  order  to  prevent 
the  craft,  when  at  rest,  from  being  blown  over  backwards  by  a  wind 
from  the  front.  The  amount  is  largely  a  matter  of  experience  but 
depends  on  the  size  and  height  above  the  water  of  the  wing  surfaces 
and  air-resisting  parts. 

*  Several  years  ago  the  author  proposed  this  feature,  and  was  the  first  to  put  it 
into  actual  practice  in  his  aeroboat,  publicly  exhibited  in  1912,  after  months  of  pioneer 
experimenting. 


141 

2.  An  excess  of  flotation  forward,  to  give  plenty  of  lift  over  on- 
coming waves,  and  to  prevent  upsetting  by  a  wind  under  the   tail. 
Ordinarily  ample  flotation  is  given  forward,  because  of  the  necessary 
forward  position  of  the  pontoon  for  hydro-planing. 

3.  Sufficient  flotation  on  either  side,  to  prevent  side  gusts  from 
pushing  a  wing  into  the  water,  the  construction  of  the  wings  being 
so  fragile,  ordinarily,  that  contact  with  the  water  may  result  in  dam- 
age.    This  side  flotation  is  usually  obtained  by  using  either  a  twin- 
float  or  a  three-float  system,  the  latter  consisting  of  a  large  central  float 
and  smaller  side  floats  placed  on  the  tips  of  the  wings      Even  where 
twin  floats  of  large  size  are  used  additional  floats  on  the  wings  are  some- 
times fitted. 

The  provision  forj  excesi  flotation,  as  indicated,  is  of  the  utmost 
importance,  since  high  winds  out  on  the  water  exert  a  most  power- 
ful force  in  tending  to  upset  the  craft  when  it  is  at  rest,  drifting  or 
anchored.  When  anchored  in  a  severe  storm,  it  has  often  happened 
that  the  wind  blowing  on  the  wings  has  lifted  the  entire  craft  bodily 
out  of  water,  upsetting  it.  In  this  connection  the  feature  of  folding 
back  the  wings,  when  on  the  water,  is  a  particularly  advantageous  one. 

Hydroplaning. 

The  action  of  a  surface  at  an  angle  of  incidence,  moved/ in  water 
or  "hydroplaning,"  is  the  same  as  "aeroplaning"  in  air,  in  that  a  Lifting 
Force  is  generated  at  the  expense  of  a  Drift  or  Resistance.  The  {plan- 
ing surface  on  pontoons  or  hulls  is  obtained  by  suitable  conforma- 
tion of  the  bottom,  the  sides  of  the  hull  causing  this  action  to  be  very 
similar  to  the  action  of  a  surface  of  the  "wetted"  area  and  shape  of  the 
bottom  on  which  the  water  is  impinging  when  immersed. 

The  various  shapes  of  the  bottom  of  aeroboat  hulls^  ojr  pontoons, 
arched,  flat,  or  double  concave  "V"  all  appear  to  have  very  nearly 
the  same  hydroplaning  power  in  lifting  force.  The  contours  and  dis- 
position of  these  planing  surfaces,  however,  differ  greatly  in  efficiency. 

In  getting  under  way,  the  marine  aerpplane,  ploughs  thru  the 
water  as  a  displacement  boat  for  some  time,  until  the  speed  thru  the 
water  becomes  great  enough  to  cause  the  hydroplaning  action  of  the 
hull  to  take  effect,  after  which,  as  the  speed  increases,  the  "planing" 
surface  lifts  more  and  more  of  the  hull  out  of  the  water,  at  the  same 
time  reducing  its  own  surface  and  resistance.  Meanwjiile,,  the  wings 
are  acquiring ,  speed  ( enougji  relative  to  the  air  to  acquire  their  lift, 
and,  finally,  the  amount  of  surface  "planing"  on  the  water  is  reduced 
to  a  fraction  of  an  inch,  and  the  speed  of  the  wings  thru  the  air  being 
sufficient  for  support,  the  craft  leaves  the  water.  In  the  .acquirement 
of  flying  speed  on  the  w,ater,|the  greates£  powejr  is  required  at  just 
that  stage  wher^  displacement  travel  ceases  and  "hydroplaning"  be- 


142 


FLAT  BOTTOM  FLAT  BOTTOM 

CfturtcK  BOW  PLUM-  xaw  BOW 


Diagrams  of  Floats  or  Pontoons,  and  air  resistance  values  —  Aeroboats  and  Pontoon 
Hydro-aeroplanes,  showing  centers  of  forces  and  balance. 


143 

gins,  and  unless  enough  power  is  available  to  overcome  the  drift  on 
the  hull,  necessary  to  obtain  this  lift,  ''planing"  will  not  be  attained. 

Any  suction  tending  to  hold  the  craft  down,  or  to  add  to  (the  hull's 
resistance,  may  render  "planing"  at  speed  difficult,  so  that  everything 
should  be  done  to  make  the  bottom  of  the  hull  or  pontoon  a  good  "planer." 

This  is  secured  primarily  by  having  a  high  aspect  ratio  to  the 
planing  area  of  the  bottom  —  as  important  in  hydroplaning  pontoons 
as  it  is  in  aeroplanes. 

So  definite  a  factor  is  this  in  determining  "planing,"  that  it  may 
be  laid  down  as  a  general  rule,  regardless  of  laboratory  results,  that 
for  every  500  Ibs.  weight  of  machine  there  should  be  at  least  one  foot 
width  of  bottom.  If  this  be  obtained  in  two  pontoons,  the  increased 
side  resistance  would  give  slightly  more  drag  than  if  a  large  central 
float  were  used,  with  the  small  side  pontoons  lifting  readily  out  of  the 
water. 

The  angle  of  incidence  of  the  flat  bottom  that  gives  the  best  re- 
sults is  about  4°  incidence;  any  greater  angle  than  this  gives  too  high 
a  resistance  and  is,  therefore,  wasteful  of  power. 

The  contour  of  the  bottom,  so  as  to  obtain  this  angle  on  the  wetted 
surface  and  with  the  area  and  center  of  lift  properly  placed,  is  worthy 
of  extensive  study. 

Centers   of  Forces   and   Balance. 

As  indicated  in  the  diagram,  the  proper  balance  for  planing' is  de- 
termined by  considering  the  thrust,  the  lift  on  the  tail  in  the  pro- 
peller stream,  the  c.  g.,  and  the  c.  h.,  or  center  of  the  hydroplaning 
pressure  on  the  bottom.  The  thrust,  being  so  high  above  the  water, 
exerts  a  powerful  I  moment  about  the  point  of  support,  i.  e.,  the  water 
surface.  This  moment  may  be  overcome  by  turning  the  tail  up,  giv- 
ing a  downward  pressure  and  moment  opposing  that  of  the  propeller. 
This  is  actually  used  at  the  very  start,  before  the  planing  ajction  on  the 
hull  is  appreciable  in  order  to  prevent  the  propeller  push  from  forc- 
ing the  bow  in  too  deep.  When  the  planing  comes  into  effect,  however, 
it  is  possible  to  do  away  with  this  negative  tail  moment,  —  which  is  both 
slowing  down  and  adding  weight  to  what  the  pontoons  must  lift  —  by 
having  the  wetted  hydroplane  surface  far  enough  forward  to  have  the 
c.  h.  in  front  of  the  c.  g. 

The   Shape   of  the   Bottom. 

The  contour  of  the  bottom  of  the  floats  must  be  such  that  the 
c.  h.  is  well  forward,  when  planing,  and  yet  with  sufficient  planing 
surface  aft  to  feather  on  the  water  and  prevent  the  craft  from  jump- 
ing back  too  easily  on  its  tail,  since  the  latter  condition,  causing  sud- 
den changes  in  the  angle  of  the  bottom  and  its  planing  pressure,  is 


144 

what  gives  rise  to  the  disagreeable  effect  of  "porpoising"  • — -a  fore  and 
aft  rocking  and  jumping,  which  is,  at  times,  difficult  to  stop. 

At  the  front  the  contour  should  be  such  that  there  is  a  large  ex- 
panse of  hydroplaning  surface  in  front  of  that  wetted  in  ordinary  opera- 
tion, in  order  to  give  ample  lift  at  the  bow  for  proper  recovery  when 
alighting  on  the  water  at  a  steep  gliding  angle  —  otherwise  the  nose  of 
the  float  might  catch  in  the  water  and  upset  the  craft. 

It  is  interesting  in  this  connection  to  point  out  a  feature  on  many 
floats  or  hulls  that  defeats  its  own  purpose.  It  is  assumed  by  many 
designers  that  a  bow  gradually  turning  up  steeply,  presenting  a  greater 
hydroplaning  angle,  will  be  the  most  effective  in  recovery  charac- 
teristics on  a  "nose  down"  landing.  As  a  matter  of  fact,  the  recovery 
moment  is  dependent  not  only  on  the  size  of  surface  and  speed  of  landing, 
but  also  on  the  lever  arm  of  this  pressure  at  the  bow,  about  the  c.  g. 
As  indicated  on  the  diagram  (the  pressures  being  normal  to  the  sur- 
face) a  flatter  angle  at  the  bow  gives  a  much  more  powerful  recovery 
moment.  This  is  fully  verified  by  actual  practice. 

Steps. 

In  order  to  break  up  the  contour  into  the  various  areas  at  dif- 
ferent positions  and  angles,  the  practice  of  building  the  bottom  in 
"steps"  is  resorted  to.  A  reduction  of  friction  resistance  and  splen- 
did effect  in  dividing  up  the  surface  is  obtained  by  this  feature,  if  the 
steps  are  made  from  two  to  five  inches  deep,  with  ample  ventilation, 
i.  e.,  large  air  tubes  or  air  slots  in  the  hull,  to  feed  air  into  the  corner 
of  the  steps,  for  the  relief  of  the  suction  created  there  by  movement 
of  the  water.* 

In  considering  the  contour  of  a  float,  the  fact  that  the  water  will 
acquire,  and  for  a  time  hold,  an  acceleration  downwards,  produced  by 
passing  under  an  inclined  surface,  is  often  lost  sight  of.  And  the  friction 
resistance  of  long  surfaces  is  very  great. 

Seaworthiness. 

Perhaps  the  most  difficult  incompatibility  (excepting  that  of 
"stability  and  controllability"  on  an  aeroplane)  is  to  make  a  hydro- 
plane type  of  hull  seaworthy.  The  fact  that  the  hull,  when  it  gets 
up  to  speed,  is  supported  by  dynamic  water  pressure,  means  that  any 
increase  or  decrease  of  angle  or  surface  wetted,  caused  by  choppy  water, 
will  result  in  terrific  bumping  and  pounding,  and  the  old  saying  about 
the  hardness  of  water,  if  hit  hard  enough,  becomes  uncomfortably 
evident.  If  the  angle  at  the  bow,  as  the  craft  goes  into  a  wave  is  very 

*  The  surprising  magnitude  of  this  suction  is  illustrated  by  the  fact,  that,  in  the 
early  development  of  hydro-aeroplanes,  a  single  J^-inch  air-tube  was  considered  suffi- 
cient ventilation  for  a  step,  which  today  would  be  required  to  have  at  least  three  2>£- 
inch  tubes. 


145 

steeply  upturned,  the  bump  is  felt  with  unusual  force,  since  it  also 
tends  to  slow  down  the  craft.  Where  a  hull  is  used  in  which  the  upturn 
at  the  bow  is  kept  as  flat  as  possible,  very  little  bumping  is  felt,  in  com- 
parison, the  hull  riding  over  the  waves  instead  of  pounding  into  them. 
However,  in  the  latter  case,  since  considerable  depth  to  the  bow  is 
necessary  to  avoid  "tripping"  on  waves,  a  freeboard  is  obtained  by  a 
"cruiser"  bow  construction  quite  readily,  or  by  use  of  a  "turtle  back" 
bow.  The  cruiser  bow  cuts  thru  very  large  waves,  throwing  a  great 
amount  of  spray  to  be  sure,  but  the  speed  of  the  craft  is  not  stopped 
as  suddenly  as  with  an  upflare  bow,  and  spray  is  readily  protected 
against. 

The  shape  of  the  bottom  is  of  importance  .for  seaworthiness,  since 
a  V  bottom  is  found  to  give  much  less  pounding,  an  easier  entry,  a 
softer  landing,  and  much  less  tendency  to  bounce  on  alighting.  In 
addition,  tendency  of  the  craft  to  skid  outwards,  when  being  turned 
on  the  water,  is  somewhat  provided  against.  Pounding  on  the  bottom 
causes  very  great  strains  on  the  seams,  by  spreading,  and  a  V  bottom 
by  relieving  this,  of  necessity  reduces  the  possibility  of  leakage. 

The  long  dragging  hull  in  the  rear,  on  some  types,  greatly  increases 
the  length  of  run  necessary  to  get  off,  because  of  its  added  and  un- 
necessary resistance.  The  hull  at  the  rear  should  be  given  a  positive 
action  that  will  lift  it  out  of  the  water,  but  this  may  become  too  great, 
resulting  at  the  start  in  digging  the  nose  in  too  deeply. 

All  these  features  require  careful  compromise  and  balance.  Sev- 
eral outlines  of  hulls  and  floats  are  given. 

Many  features,  such  as  self -bailing  cockpits,  and  thorough  water 
protection  of  the  motor,  etc.,  require  attention  for  increased  sea-worthi- 
ness. 

But  the  most  seaworthy  characteristic  of  marine  aeroplanes  has 
been,  and  possibly  always  will  be,  ability  to  rise  out  of  the  water  quickly 
and  with  the  shortest  run. 

The  greater  excess  of  flotat'on  of  the  aeroboat  type  is  a  feature 
of  considerable  importance.  On  a  marine  aeroplane,  consisting  of  a 
land  machine  mounted  on  pontoons,  a  very  large  pontoon  at  the  rear  is 
required,  to  give  anywhere  near  the  excess  of  flotation  obtained  with 
the  aeroboat.  In  this  —  and  in  the  greater  ease  with  which  the  centers 
of  flotation,  hydroplaning,  thrust  and  c.  g.,  may  be  brought  closer 
together  —  there  are  found  the  only  real  advantages  of  the  "boat"  type 
over  the  "hydro"  since  flying  characteristics  and  even  "planing,"  on 
either  one,  are  governed  by  the  same  limitations.  Structurally,  the 
aeroboat  type  can  be  built  stronger  for  the  same  weight  than  a  "hydro," 
or  pontoon  aeroplane,  and  when  the  great  stresses  induced  by  "side 
swiping"  in  landing  across  wind  are  considered,  the  boat  is  decidedly 
advantageous  in  being  so  well  self-contained. 


146 

The  relative  merits  of  the  single  pontoon  and  twin  pontoon  sys- 
tems are  not  yet  well  defined.  The  single  pontoon  is  handier  in  a 
sea,  but  twin  pontoons,  on  a  large  craft,  give  a  wider  expanse  of  bot- 
tom, thereby  improving  the  planing  by  a  higher  "aspect  ratio,"  but 
at  the  expense  of  more  frictional  resistance.  The  twin  pontoon  system 
is  apparently  well  adapted  to  launching  devices. 

Elements  of  seaworthiness  found  in  the  larger  sized  marine  aero- 
planes are  distinctly  advantageous,  and  indicate  that  for  real  work  in 
the  open  sea,  seaplanes  will  become  huge  in  size,  and  will  have  to  pos- 
sess great  range  of  action  and  excess  of  power. 


Above  left  —  The  Burgess-Dunne  Seaplane,  pusher  type  with  pontoons. 
Above  right  —  Martin  Tractor  Seaplane,  shown  also  at  lower  left. 

Lower    right  —  Loening    Monoplane    Aeroboat,    an    early    experimental    marine 
aeroplane,  the  first  of  the  flying  boat  class. 


CHAPTER    XII. 
FLYING,   STABILITY  AND   AIRWORTHINESS. 

The  characteristics  of  resistance,  lift,  speed,  and  power  of  the 
aeroplane  having  been  studied,  and  attention  having  been  given  to  the 
construction  and  adjustment  of  these  machines,  it  is  appropriate  now 
to  consider  the  actual  flying  of  the  machine. 

As  already  outlined,  it  must  be  borne  in  mind  that  the  aeroplane 
is  supported  in  a  perfectly  free  fashion  on  a  medium  that  is,  at  times, 
very  treacherous,  and  the  most  efficient  aeroplane  in  the  world,  as 
to  speed  and  power,  and  the  very  best  and  refined  in  construction,  is 
more  or  less  worthless  unless  it  embodies  "controllability"  and,  above 
all  "airworthiness." 

For  the  military  aviator,  the  importance  of  acquiring  a  very  sound 
and  intelligent  grasp  of  the  principles  of  stability  and  operation  involved 
in  the  notion  "airworthy,"  cannot  be  overestimated. 

Actual  instruction  in  the  manipulation  of  controls  on  the  ma- 
chines, thorough  practice  in  acquiring  the  "feel"  of  the  air,  and  de- 
velopment of  unerring  judgment  on  landings,  form  the  major  part 
•of  the  practical  work  in  the  training  of  aeroplane  pilots.  But  unless 
this  is  accompanied  by  an  intelligent  understanding  of  the  actions 
of  aeroplanes  in  the  air,  the  pilot  is  little  more  than  a  somewhat  in- 
stinctive automaton. 

No  mathematics,  or  formulae,  need  be  involved  in  the  consid- 
eration of  the  stability  and  operation  of  aeroplanes.  But  there  is 
required  a  continued  and  judicious  use  of  "common  sense." 

The  subject  may  be  divided  into  the  three  broad  generalities  of 
considering : 

1.  The  flying  of  the  machine,   the  assuming  of    different   atti- 
tudes, unsafe  positions  that  may  be  taken,  and  proper  methods  of  oper- 
ation. 

2.  The  stability  of  the  machine,  which  may  at  once  be  defined 
as  the  degree  and  manner  in  which  the  aeroplane  tends  of  its  own  accord 
to  keep  a  certain  relative  "even  keel"  attitude  to  the  air  stream. 

3.  The  airworthiness  of  the  machine,  or  degree  in  which    com- 
fortable  stability  is  obtained  without  too  much  sensitiveness    to   air 
disturbances,  and  controllability,  is  obtained  without  making  the  aero- 
plane too  easy  to   upset. 


148 

The  absolute  opposition  of  inherent  stability  to  controllability  is 
always  met  in  flying  characteristics,  and  it  is  a  fact  that  an  inherently 
stable  and  safe  aeroplane  is  stiff  and  apt  to  "fight"  its  controls,  while 
it  is  sensitive  to  and  moved  by  air  disturbances  —  whereas  a  "neutral" 
stability  aeroplane,  with  powerful  controls  and  no  tendency  to  hold 
any  position  relative  to  the  air,  is  handy  and  precise  in  answering  its 
helm,  and  is  not  readily  upset,  if  equipped  with  a  good  automatic  pilot 
mechanism. 

The  popular  notion,  held  by  many  intelligent  people,  that  "sta- 
bility" means  "steadiness  in  flight"  is  very  erroneous.  The  least  air 
disturbance  causes  a  "stable"  aeroplane  correspondingly  to  adjust 
itself  to  keep  the  same  attitude  relative  to  the  air  so  that  its  position 
relative  to  the  ground  is  changed  by  air  movements,  and  so  percep- 
tibly that  on  a  rough,  puffy  day  an  inherently  stable  aeroplane  ap- 
pears to  roll,  pitch  and  sway  in  a  most  alarming  fashion,  while,  as  a 
matter  of  fact,  it  is  merely  answering  to  the  air  billows.  It  is  much 
more  correct  to  conceive  of  an  "inherently  stable"  aeroplane,  pri- 
marily as  "non-capsizable,"  and  not  at  all  steady  in  its  flight.  For 
that  reason  a  neutral  stability  aeroplane,  with  a  delicate  mechanical 
automatic  pilot,  makes  a  much  steadier  gun  platform. 

CENTERS   OF   FORCES. 
The  aeroplane,  in  flight,  is  subjected  to  the  action  of  four  forces: 

(1)  The  Thrust  —  acting  at  the  center  of  thrust,  C.  T.  —  which  is 
merely  the  line  of  the  propeller  axis. 

(2)  The  Total  Resistance  —  acting  at  the  center  of  resistance, 
C.  R.  —  which  is  determined  by  balancing  the  air  resistances  of  all  the 
separate  structural  parts  (see  Chap.  IV)  with  the  drift,  and  finding 
the  resultant  point  at  which  a  force  equivalent  to  the  total  resistance 
would  be  applied. 

(3)  The  Lift  —  acting  at  the  center  of  pressure,  C.  P.  —  which 
is  the  center  of  pressure  of  the  lifting  forces  for  the  particular  angle 
of  incidence,  at  which  flying  is  taking  place,  and  found  from  the  sur- 
face section  data  and  tail  lift  data. 

(4)  The  Weight  —  acting  at  the  center  of  gravity,  C.  G. 

Center  of   Gravity. 

It  is  of  fundamental  importance,  before  studying  this  subject 
further,  to  know  how  and  where  the  center  of  gravity  of  any  machine 
is  located. 

An  aeroplane  is  suspended  in  the  air  and  rotates  about  its  center 
of  gravity,  so  that  it  is  proper  to  consider  the  path  of  the  center  of  gravity, 
in  considering  the  trajectory  of  any  machine.  An  aeroplane  distinctly 
does  not  rotate  about  any  center  of  lift  or  resistance. 

The  center  of  gravity,  therefore,  must  be  known,  and  should  be 
measured  and  marked  on  the  machine. 

The  manufacturer  furnishes  drawings  and  data,  indicating  the 
proper  position  of  the  center  of  gravity.  The  aeroplane  user,  after  fully 


149 

loading  the  machine  for  flight,  should  determine  whether  or  not  the 
weight  of  the  machine  is  properly  balanced. 

There  are  several  methods  of  finding  the  center  of  gravity.* 

(1)  The  machine  could  be  swung,  by  flexible  suspension  from 
an  overhead  point,  and  a  plumb  line  dropped  from  this  point,  would 
intersect  the  body  at  the  c.  g.,  no  matter  what  the  position  of  the 
machine. 

(2)  The  machine  could  be  supported  on  a  large  pipe,  or  knife 
edge,  and  moved  until  balanced  on  either  side.     The  c.  g.  fore  and 
aft,  and  side  to  side,  may  be  obtained  readily  by  this  method,  although 
it  is,  at  times,  awkward  to  support  a  machine  in  this  way.   In  this  method 
the  height  of  the  c.  g.  above  the  bottom  of  the  body  is  not  so  easily 
obtained,  and  the  total  weight  is  not  measured. 

(3)  The   method   of  moments  —  in   which   the   measurement   of 
weight  is  made  at  any  two  points,  and  the   distance  between    them 
measured.     The  total  of  the  weight  at  any  two  points  of  support  is 
the  total  weight  of  the  machine,  and  as  indicated  on  the  diagram,  p.  152, 
the  center  of  gravity  is  very  easily  obtained  by  solving  the  suitable 
lever  arms.     This  is  an  exceedingly  quick,  simple  and  accurate  method 
for  a  combined  determination  of  the  weight  and  balance,  and  at  any 
large  aviation  field,  where  platform  scales  are  available,  this  method 
is  particularly   convenient.     To   determine  the  lateral   correctness  of 
the  c.  g.,  it  is  merely  necessary  to  see  if  weights  measured  at  either 
end  strut,  lifting  the  machine  about  the  opposite  wheel,   are  equal. 
The  lateral  c.  g.,  however,  is  rarely  variable  enough  to  require  check- 
ing.    To  determine  the  actual  height  of  the  c.  g.  above  the  wheels, 
it  would  be  necessary  to  repeat  the  operation  for  the  horizontal  bal- 
ance, with  the  tail  very  low  and  the  front  as  high  as  possible,  thus  es- 
tablishing an  intersection  where  the  two  c.  g.  lines  cross  each  other. 
Or,  if  the  chassis  permits,  the  machine  may  be  tilted  up  at  the  rear, 
until  a  balance  is  obtained  over  the  axle,  and  by  projecting  a  plumb 
line  above  this  an  intersection  point  is  also  obtained. 

The  longitudinal  position  of  the  c.  g.  is  the  important  one,  and 
consideration  of  the  accompanying  diagram  shows  that  if  the  total 
weight  is  reasonably  well  known,  the  single  measurement  of  the  weight 
carried  by  the  tail  skid,  and  its  distance  from  the  axle,  will  at  once 
determine  how  far  back  of  the  axle  the  c.  g.  is  situated.  For  this  only 
a  200  Ib.  spring  balance  is  necessary  in  the  field,  and  data  on  the  cor- 
rect weight  the  tail  skid  should  carry  is  given  by  the  manufacturer. 


*  It  is  necessary  to  note  that  correct  results  in  balancing  are  apt  to  be  upset,  if 
a  draught  or  wind  blows  on  the  aeroplane,  when  being  balanced.  Still  air  is  a  prere- 
quisite. 


150 

The   Equilibrium   of  the   Forces. 

These  four  forces  of  Thrust,  Resistance,  Lift  and  Weight,  acting 
at  their  respective  centers,  must  be  in  equilibrium  when  the  machine 
is  in  steady  flight. 

Generally,  an  aeroplane  is  so  designed  that  the  line  of  thrust  passes 
very  nearly  thru  the  center  of  resistance  and  the  center  of  gravity  is 
made  in  line  with  the  center  of  pressure.  The  aeroplane  is  then  said 
to  be  balanced  on  the  principle  of  "coincident  centers"  (centres  con- 
fondus).  But  there  are  notable  exceptions  to  this  practice.  For 
reasons  of  handiness  in  taking  proper  angles,  as  later  explained,  the 
center  of  thrust  is  often  placed  below  the  center  of  resistance.  This 
couple,  tending  to  turn  the  machine,  as  indicated  on  the  diagram, 
is  overcome  by  the  couple  obtained  by  having  the  center  of  lift  in  back 
of  the  center  of  gravity.  This  can  be  obtained  either  by  having  the 
center  of  pressure  of  the  surface  slightly  back  of  the  c.  g.,  or  by  intro- 
ducing a  small  lifting  force  on  the  tail.  We  are  at  once  led  then  to 
consider, 

The   Effect  of  Tail   Lift   on   the   Center  of  Lift. 

Up  to  now  we  have  considered  that  the  center  of  the  lifting  forces 
on  the  machine,  was  found  at  the  center  of  pressure  of  the  main  wings. 
This  is  only  true  if  the  tail  surfaces  are  perfectly  neutral,  as  found 
in  the  majority  of  well  balanced  aeroplanes.  If  the  tail  surfaces  re- 
ceive a  negative  pressure  —  a  downward  air  force  —  the  center  of  lift  of 
the  aeroplane  will  be  in  front  of  the  center  of  pressure  of  the  main  wing, 
and  if  the  tail  actually  exerts  a  lift,  then  the  center  of  lift  will  be  pro- 
portionately behind  the  c.  p.  of  the  wings  —  so  that  the  lift  of  the  tail 
X  its  lever  arm  back  of  the  resultant  center  of  lift  =  the  lift  of  the  wings 
X  the  lever  arm  of  the  wing  c.  p.  about  the  resultant  center  of  lift. 
This,  then,  is  the  nature  of  the  position  of  the  Total  Lift  force  (tail 
+  wings)  acting  at  the  center  of  pressure,  C.  P.,  of  the  entire  machine, 
and  is  the  point  referred  to,  ;n  considering  the  four  forces  in  equilibrium. 

Lateral   and    Directional   Centers. 

There  are  two  other  centers  to  be  considered.  The  center  of  sup- 
port, or  pressure,  may  shift  slightly  laterally  as  the  aeroplane  takes 
different  positions  in  the  air,  due  to  differences  in  the  lift  of  either  wing. 
Attention  is  given  to  this  under  "rolling." 

The  aeroplane,  with  fins,  covered  body  and  wheels,  rudders,  etc., 
presents  a  sidewise  expanse  of  surface  to  the  air.  It  is  necessary  to 
know  the  position  of  the  center  of  surface  of  all  this  side  area.  Of 
course,  the  areas  can  be  computed  and  the  center  of  area  determined, 
but  it  is  much  easier  to  cut  out  a  paper  pattern  to  scale,  of  the  side  ele- 
vation of  the  aeroplane,  and  then  by  balancing  this  on  a  pin  point, 


151 

finding  the  center  of  gravity  of  the  paper.  The  center  of  side  sur- 
face, or  directional  center,  as  it  is  sometimes  called,  may  then  be  taken 
as  slightly  in  front  of  this  point,  and  may  be  marked  on  the  machine. 

The  centers  having  been  defined,  we  are  free  to  proceed  with  the 
study  of  the  relative  movements  of  the  aeroplane  and  the  air.  Their 
classification  into,  Pitching,  Yawing,  Rolling,  has  already  been  out- 
lined on  p.  13. 

The  Moments  of  Inertia  of  the  aeroplane  about  the  various  axes 
must  also  be  considered,  since  the  inertia  largely  governs  the  rate 
with  which  a  machine  responds  to  changes  in  attitude,  with  reference 
to  the  ground.* 

CHARACTERISTICS  OF  PITCHING  OR  LONGITUDINAL  MOTION. 

The  longitudinal  motion  of  an  aeroplane  in  rising  or  descending, 
corresponding  to  changes  in  the  angle  of  incidence,  is  controlled  by 
the  elevator,  but  is  subject  to  inherent  effects  in  the  aeroplane  itself, 
due  to  the  disposition  of  surfaces  and  the  magnitude  of  the  longitu- 
dinal inertia. 

The  action  of  an  elevator  in  merely  steering  the  machine  up  or 
down,  in  its  trajectory,  is  remarkably  powerful,  and  only  a  fraction 
of  a  degree  change  in  the  angle  of  the  flaps  is  sufficient  in  normal  fly- 
ing, to  direct  the  machine  to  a  different  angle  and  path.  For  any  one 
speed  there  is  only  one  elevator  setting  and  balance,  corresponding 
to  the  one  particular  angle  of  incidence  for  that  speed,  and  any  change 
in  the  elevator  manipulated  by  hand,  changes  the  incidence,  and  for 
the  same  initial  speed  will  cause  the  machine  either  to  climb  or  point 
downwards.  Flight  at  the  different  angles  is  effected  by  changes  in 
speed  obtained  by  throttling  of  the  engine,  combined  with  a  more 
or  less  unconscious  setting  of  the  elevator  to  give  the  proper  balance. 
The  necessity  of  introducing  lifts  or  depressions  by  the  elevator,  to 
keep  the  relation  of  the  center  of  lift  about  the  c.  g.,  in  the  form  that 
will  balance  the  action  of  the  centers  of  thrust  and  resistance,  is  al- 
ways present  for  flight  at  any  angle  of  incidence.  The  pitching  control 
can  be  varied  greatly  in  delicacy  and  power  by  alterations  of  the  size, 
movement  and  leverage  of  the  elevator  flaps.  The  general  charac- 
teristics of  this  control,  however,  and  the  movements,  positions  and 
limits  of  equilibrium  longitudinally  are  common  to  all  aeroplanes. 
The  angle  ranges  that  have  been  studied  in  Chap.  VIII  now  assume  a 
more  particular  significance. 

1.  There  is  a  "normal  flight"  position  of  the  aeroplane  —  gen- 
erally when  the  body  axis  is  in  the  line  of  flight  —  where  there  is  the 
desired  combination  of  speed,  power,  glide  and  climb  characteristics. 

*  It  must  be  borne  in  mind  that  inertia  effects  tend  to  keep  the  machine  in  what- 
ever state  of  position,  motion,  or  rest,  it  happens  to  be.  And  the  greater  the  distance 
separating  items  of  weight,  the  greater  is  the  moment  01  inertia,  and,  therefore,  the 
slower  the  oscillations. 


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STABILITY   DIAGRAMS 
In  the   negative   and   lifting   tail   diagrams,   the   total    C.  P.  is  at   the   C.  G. 


153 

2.  There  is  a  "low  angle,"  or  "vol  pique,"   position,  generally 
corresponding  to  the  attitude  for  highest  speed  and  least  angle  of  in- 
cidence at  which  the  machine  is  said  to  fly  "tail  high." 

3.  There  is  a  "high  angle,"  or  "vol  cabre,"  position,  correspond- 
ing to  the  attitude  for  slowest  speed  and  large  angle  of  incidence,  at 
which  the  machine  is  said  to  fly   "tail  low." 

The  "regime"  of  flight,  at  different  speeds  and  angles,  is  all  the 
way  from  the  "tail  high"  to  the  "tail  low"  position.  At  any  of  these 
positions,  governed  longitudinally  by  the  elevator  and  the  throttle, 
the  machine  has  a  certain  climb,  depending  on  the  excess  of  the  Power 
Available  over  the  Power  Required,  a  glide  governed  by  the  total 
resistance,  and  a  certain  fuel  consumption,  all  as  outlined  in  Chap. 
VIII.  But  in  its  path  through  the  air,  if  the  machine  does  climb  or 
glide,  it  must  always  be  borne  in  mind  that  the  angle  of  incidence  is 
the  angle  between  the  chord  and  the  flight  path.  Just  because  a  ma- 
chine is  pointed  up  very  steeply,  does  not  mean  that  it  will  necessarily 
climb  steeply,  since  it  might  have  much  more  excess  power  at  a  very 
much  lower  angle,  and  actually  climb  more  feet  per  minute  by  the 
application  of  this  excess  power,  with  the  machine  on  an  apparently 
level  keel.  As  a  corollary,  a  machine  does  not  necessarily  glide  best 
the  more  it  is  pointed  down  and  speeded  up.  By  holding  a  machine 
apparently  pointed  up,  the  actual  glide  slope  would  be  flattest  if  the 
particular  high  angle  of  incidence,  corresponding  with  this  attitude 
to  the  air  flow,  was  actually  the  angle  for  best  glide.  Speeding  up  a 
machine  by  nosing  down  on  a  glide  may  increase  the  Total  Resistance 
so  much  as  to  cause  the  glide  to  steepen,  greatly.  All  these  charac- 
teristics may  be  studied  from  the  Power  and  Resistance  charts,  and  any 
aviator  can  profitably  acquire  familiarity  with  them. 

Although  its  significance  is  often  overestimated,  consideration 
of  the  "reversed  flight"  region  may  be  given  here.  Referring  to  p.  96, 
it  is  seen  that  at  speeds  below  angles  of  10°,  increase  of  angle  of  inci- 
dence, corresponding  to  slower  speeds,  involves  a  pronounced  rise  in 
the  Power  Required,  due  to  increased  resistance.  And  above  17° 
the  lifting  power  of  the  wings  actually  decreases.  The  effect  of  flying 
at  these  high  angles  is  to  ause  an  inversion  of  controls.  Increasing 
the  angle  of  incidence,  whn  in  horizontal  flight,  without  giving  the 
engine  "more  throttle,"  actually  causes  the  machine  to  sink,  and  whereas 
if  flying  at  14°,  let  us  say,  the  machine's  incidence  were  to  be  reduced 
to  10°,  with  the  same  engine  power,  the  maneuver  would  result  in 
a  climb,  due  to  gain  in  excess  power.  The  old  conception,  then,  of 
pointing  a  machine  up  for  climbing,  and  down  for  gliding,  is  not  al- 
ways correct;  and  the  aviator  at  some  angles  may,  much  to  his  surprise 
find  himself  climbing  when  he  points  the  machine  down,  and  sinking 
when  he  points  it  up  for  a  climb.  Such  phenomena  are  only  too  often 
blamed  on  "puffs  and  uptrends,"  when,  as  a  matter  of  fact,  a  glance 


154 

at  the  power  chart  would  show  the  reason  for  apparent  inversions  of 
this  kind. 

This  leads  at  once  to  the  realization  that  higher  power  is  often 
of  advantage  in  attaining  slower  speeds,  since  flying  can  then  be  done 
at  angles  where  the  resistance  would  prove  too  much  for  a  lower-powered 
machine.  Thus,  referring  to  p.  96,  it  is  seen  that  the  slow  speed  attain- 
able at  800  r.  p.  m.  is  43  miles  an  hour,  whereas  an  increase  to  1400 
r.  p.  m.  would  very  likely  permit  of  flying  at  39  miles  an  hour.  The 
"regime  lente"  or  "slow,"  at  which  the  aeroplane  is  flying,  in  cabre  atti- 
tudes, is  perhaps,  the  most  difficult  one  to  negotiate,  and  there  are  not 
many  pilots  expert  enough  to  get  the  very  slowest  speeds  out  of  their 
machines.  Flying  at  the  high  speeds  is  merely  a  matter  of  giving  the 
engine  all  the  power  it  has,  and  being  on  the  alert  for  the  uncomfortable, 
quicker  action  of  puffs.  Flying  a  machine  at  its  attitude  for  best 
climb,  or  best  glide,  is,  of  course,  a  matter  of  systematic  practice,  but 
information  from  the  Power  Chart  is  particularly  of  value  for  this. 


Stalling   and   Diving. 

An  aeroplane's  angle  of  incidence  can  be  increased  or  decreased, 
providing  the  speeds  are  changed  in  proportion.  But,  at  any  angle, 
if  the  speed  drops  the  aeroplane  is  subject  to  loss  of  headway,  and  con- 
sequently to  loss  of  support.  This  condition  is  called  a  "stall."  There 
are  many  ways  in  which  an  aeroplane  can  be  stalled,  either  by  a  false 
maneuver  or  a  peculiar  air  disturbance.  Immediately  after  a  machine 
has  lost  headway,  however,  it  begins  to  sink,  either  sideways,  tail  first, 
or  on  a  level  keel.  The  latter  condition  corresponds  merely  to  a  sud- 
den rise  in  the  angle  of  incidence,  and  recovery  is  possible.  In  the 
other  two  conditions,  speed  for  flight  can  be  regained  only  after  a  long 
fall,  and  then  only  if  the  machine  has  the  necessary  recovery  charac- 
teristics and  the  pilot  the  presence  of  mind  to  apply  them. 

Stalling  on  turns  is  considered  later.  Stalling  due  to  pitching 
maneuvers  may  be  considered  here. 

On  a  steep  climb,  continual  incidence  increase  and  slowing  down 
may  eventually  result  in  exceeding  the  maximum  lifting  angle,  and 
the  condition  of  lost  support,  due  to  too  great  an  angle  and  lack  of 
speed,  is  merely  a  stall.  Novices,  when  leaving  the  ground,  often  point 
their  machines  up  too  steeply  and  thereby  lose  headway,  necessary 
for  support. 

Pancaking  is  usually  taken  to  define  the  settling  of  a  machine, 
on  landing,  due  to  having  turned  up  so  steeply  that  a  stall  is  reached, 
and  the  machine  robbed  of  speed  and  support  sinks  more  or  less  ab- " 
ruptly  to  the  ground,  with  little  if  any  forward  speed. 


155 

The  acquirement  of  the  proper  skill  in  operation  to  feel  an  ap- 
proaching stalled  condition  and  to  be  in  such  position  that,  if  the  power 
suddenly  ceases,  a  tendency  to  stall  may  immediately  be  overcome 
by  taking  a  proper  gliding  angle,  is  better  taught  by  experts  in  flight, 
on  any  particular  machine. 

In  taking  a  downward  path  an  aeroplane  may  be  gliding  —  float- 
ing down  on  a  long  slope,  held  at  a  certain  incidence,  and  therefore 
coming  down  at  a  constant  speed  —  or  it  may  be  diving,  i.  e.,  coming 
down  on  what  is  practically  a  fall  with  no  particular  value  to  the  inci- 
dence, and  constantly  increasing  speed. 

Any  of  the  foregoing  kinds  of  stalling  may  be  met  with  as  easily 
by  sudden  changes  in  wind  direction,  due  to  up  or  down  trends,  as 
by  changes  in  the  attitude  or  speed  of  the  aeroplane. 

Stalling,  when  on  the  gliding  path  or  slope,  is  a  frequent  and  lit- 
tle appreciated  source  of  accident.  The  fact  is  lost  sight  of,  only  too 
often,  that  angles  of  incidence  are  just  as  important  on  the  down- 
ward slope  in  a  glide,  as  in  horizontal  flight,  and  stalling  may  be  reached 
by  a  gradual  loss  of  headway  and  increase  of  angle.  After  a  long  dive, 
a  mistake  in  recovery  to  more  level  flight  may  result  in  a  stall.  It  has 
frequently  happened  that  aeroplanes  with  a  small  longitudinal  inertia 
(resembling  the  old  type  "pusher"  aeroplanes)  have  been  turned  up 
too  quickly  after  a  dive  or  glide  so  that  the  tendency  of  the  machine 
to  keep  on  going  in  the  direction  of  the  dive,  for  a  moment  results  in 
the  wings  attacking  the  air  at  a  very  high  angle  of  incidence  and  a 
quickly  retarded  speed.  A  characteristic  of  this  kind  is  due  largely 
to  so  small  a  longitudinal  moment  of  inertia,  that  the  aeroplane  could 
be  turned  around  its  transverse  axis,  without  much  displacement  of 
its  center  of  gravity,  and  is  distinctly  a  dangerous  one.  Lack  of  knowl- 
edge on  this  feature  has  cost,  doubtless,  many  lives. 

The  usual  pitching  control  of  pushing  forward  on  a  post  to  go 
down  and  pulling  back  to  increase  the  angle,  is  very  instinctive. 

The  general  construction  of  the  elevator  surfaces  on  aeroplanes, 
in  the  form  of  a  large  fixed  area  to  which  trailing  flaps  are  attached, 
gives  rise  to  an  interesting  phenomenon. 

In  flying  at  very  high  angles  of  incidence,  since  the  entire  machine 
is  inclined,  it  follows  that  the  fixed  tail  surface  has  a  high  angle 
of  incidence  to  the  air  flow  past  it,  and  therefore  is  subject  to  a  Lift. 
In  the  preservation  of  the  balance,  it  may  happen  where  the  fixed 
portion  is  large  and  the  flap  small,  that  the  flap  is  turned  to  a  con- 
siderable angle  upwards.  The  air  flows  past  the  inclined  front  fixed 
surface,  and  breaks  up  into  eddies,  seriously  interfering  with  the  air 
pressure  on  the  flap,  with  the  effect  that  a  further  up-turn  to  the  flap 


156 

would  result  in  no  appreciable  change  in  the  air  forces.  In  other  words 
the  pitching  of  the  machine  would  lack  any  response  to  the  elevator 
movement,  due  to  the  masking  of  the  air  on  the  flaps  by  the  fixed  plane 
in  front  of  them.  If  an  action  like  this  were  to  take  place  at  about  11° 
incidence,  on  an  aeroplane  that  would  not  stall  before  12°  to  14°  had  been 
reached,  it  would  obviously  be  impossible  in  flying  level  to  stall  such  a 
machine  by  pulling  the  elevator  control  to  its  limit.  This  feature,  though 
in  a  sense  a  serious  limiting  factor  in  controllability,  has  actually  been 
used  on  many  aeroplanes  for  training  purposes,  with  excellent  success, 
as  a  "safety  factor"  against  stalling. 

Steep  dives  introduce  other  dangers  than  those  already  indicated 
as  due  to  the  possibility  of  stalling  on  recovery.  The  most  import- 
ant effect  of  a  steep  dive  is  the  acquirement  of  very  greatly  increased 
velocity,  which  may  prove  exceedingly  dangerous,  due  to  the  tail  effect 
considered  below,  and  due  to  the  possibilities  of  great  strains  on  the 
machine. 

Different  types  of  aeroplanes,  of  course,  vary  widely  in  their  pitch- 
ing characteristics,  but  in  practically  all  aeroplanes  with  deeply  cam- 
bered surfaces,  at  low  angles  of  1°  or  2°  flying  becomes  exceedingly 
uncomfortable.  The  machine  apparently  loses  much  of  its  handiness 
in  the  control  of  pitching,  because  the  surfaces  at  these  low  angles 
are  flying  at  low  values  of  KL,  and  greatly  varying  values  of  L/D, 
so  that  slight  variations  in  the  wind  direction  cause  rather  large  and 
sudden  changes  in  the  pressures,  and  consequent  "jumping"  of  the 
machine.  Large  surfaced,  deeply  cambered  machines,  with  any  con- 
siderable excess  power,  always  exhibit  this  characteristic  when  flown 
with  full  power  on  the  horizontal.  And  the  angle  on  some  sections 
may  come  so  perilously  near  to  the  angle  of  no  Lift,  and  rear  most 
c.  p.  position,  as  to.  introduce  the  danger  of  a  sudden  dive,  which  the 
elevator  may  not  be  big  or  powerful  enough  to  negotiate.  Aeroplanes 
with  deeply  curved  wing  sections  and  an  excess  of  power  for  climb, 
should  never  be  flown  on  the  horizontal,  with  "all  power  out,"  if  this 
low  angle  region  is  approached  thereby. 

Longitudinal   Stability. 

The  attitudes  assumed  and  limits  of  control  reached  in  pitching 
having  been  considered  briefly,  attention  may  be  given  to  the  natural 
characteristics  of  the  longitudinal  equilibrium  of  aeroplanes  quite  in- 
dependent of  manually  controlled  pitching. 

The  center  of  pressure .  on  practically  every  type  of  aeroplane 
surface  extensively  used  at  present,  excepting  the  "Taube,"  moves 
backward  as  the  incidence  is  decreased,  and  forward  as  the  incidence 
is  increased,  within  the  ordinary  range  of  flight  angles  of  0°  to  12° 
(see  Chap.  VII).  This  means  that  the  main  lifting  force  has  a  pro- 


157 

nounced  tendency  to  make  a  machine  dive  still  more  when  the  angle 
of  incidence  is  decreased,  and  to  stall  when  the  angle  is  increased.  This 
is,  clearly,  a  condition  of  instability,  i.  e.,  any  pitching  is  accentuated 
by  the  air  pressure.  For  stability  —  that  is,  a  tendency  for  the  ma- 
chine to  right  itself  —  it  would  be  necessary  to  have  the  center  of  pres- 
sure move  back  on  increase  of  angle,  and  move  forward  on  decrease 
of  angle.  Many  attempts  have  been  made  to  attain  this  by  the  use 
of  reversed  curve  sections  of  wing,  but  up  to  now  they  have  all  been 
at  a  great  sacrifice  of  efficiency.  A  c.  p.  position  that  is  almost  sta- 
tionary, thru  the  range  of  angles,  has  been  closely  approached  by  some 
of  the  newer  flat  section  wings,  and  by  use  of  a  washout  in  the  angle 
or  the  upturned  tip  as  in  the  "Taube."  But  for  actual  positive  stabil- 
izing action,  it  has  been  necessary  to  rely  on  the  action-  of  a  tail  plane  or 
auxiliary  surface. 

This  brings  us  to  the  consideration  of,  perhaps,  the  most  import- 
ant and  essential  inherent  stability  characteristic  of  an  aeroplane  — 
the  powerful  corrective  action,  on  disturbances  of  longitudinal  equi- 
librium of  the  convergent  tandem  arrangement  of  surfaces. 

The  definitions  of  the  convergent  tandem  system,  often  called  the 
"longitudinal  dihedral,"  which  is  so  desirable  for  longitudinal  stability, 
and  sketches  of  several  systems  of  tail  and  main  surface  combinations, 
are  given  in  the  accompanying  diagram. 

For  most  practical  purposes,  on  the  average  present  day  aero- 
plane, the  complete  tail  surface,  situated  at  about  three  chord  lengths 
from  the  main  surface,  is  made  to  have  an  area  of  about  l/6th  of  the 
main  surface.  This  is  inclusive  of  the  flaps,  which  are  merely  a  means 
of  altering  the  camber  and  pressures  on  the  tail  surface  for  purposes 
of  control.  The  error  should  not  be  made  of  considering  the  fixed  tail 
pieces  as  separate  from  the  flaps,  because  of  the  continuity  of  the  two, 
except  in  the  extreme  case  of  "masking  '  already  considered.  The 
main  and  auxiliary  surfaces  could  be  of  various  different  proportions, 
such  as  the  tandem  disposition  of  equal  surfaces,  as  in  the  old  Langley 
machines,  or  the  "Canard"  arrangement  with  the  smaller  surface  in 
front  (see  p.  152). 

Whatever  the  relative  size  of  the  surfaces,  if  the  angle  of  the  front 
one  is  positive  and  the  angle  of  the  rear  one  negative,*  the  system 
is  said  to  be  a  "convergent  tandem";  and  its  characteristic  is  that 
when  the  angle  of  incidence  of  the  aeroplane  is  decreased,  the  air  force 
on  the  tail  becomes  more  negative,  acting  downwards,  thus  tending  to 
force  the  nose  of  the  machine  up,  while  if  the  aeroplane  assumes  a  cabre 
position,  the  rear  surface  lifts  more,  thus  pointing  the  nose  of  the  ma- 
chine down.  'This  action  is  accentuated,  in  addition,  by  the  slowing 

*  In  all  this  discussion  the  angle  of  the  tail  surfaces  with  the  air  is  meant,  i.  e., 
interference  of  the  air  flow  by  the  main  surface  is  taken  account  of  by  the  usual  reduc- 
tion of  a  degree  or  two. 


158 

down  of  the  machine  at  high  angles  and  the  speeding  up  at  low  angles. 
Practically  all  Lift  values  on  aerofoils  increase  at  a  much  steeper  rate 
at  low  angles  than  at  high  angles.  So  that  the  rear  surface,  as  in  the 
divergent  tandem,  will  actually  change  its  Lift  in  less  proportion  than 
the  front  one,  for  changes  in  incidence,  —  and  this  accentuates  the  action 
of  the  pressures  on  the  main  surface  alone,  tending  to  make  the  machine 
nose  over  still  further  of  its  own  accord  when  it  pitches  forward,  and 
to  make  it  nose  up  still  more  when  the  incidence  increases.  The  ''di- 
vergent tandem,"  then,  is  naturally  an  unstable  system.  The  con- 
vergent tandem  is  often  spoken  of  as  a  "longitudinal  dihedral,"  because 
the  surfaces  are  turned  up  relative  to  each  other. 

It  is  not  always  necessary  to  have  a  negative  tail  in  order  to  obtain 
the  desirable  pitching  stability,  since  the  main  surface  may  be  set  at 
+  3°  and  the  tail  surface  at  +  2°,  with  an  interference  on  the  tail  caus- 
ing a  1°  negative  flow,  which  would  give  a  longitudinal  dihedral  of  2°, 
and  still  leave  the  tail  a  lifting  one,  at  an  incidence  to  the  air  of  1°. 

This  leads  to  the  consideration  of  the  effect  on  the  balance  of  speed 
variation  of  the  air  passing  the  tail  surfaces.  Varying  the  r.  p.  m. 
of  the  propeller  by  the  throttle  varies  the  speed  of  the  air  thrown  back 
by  the  blades.  In  every  type,  excepting  the  "torpedo"  type,  the  pro- 
peller is  in  front  of  the  tail  surfaces,  and  therefore  changes  in  the  pro- 
peller stream  affect  the  pressures  on  the  tail.*  In  machines  with  a 
neutral  tail,  neither  negative  nor  lifting,  the  effect  of  stopping  or  speed- 
ing up  the  propeller  is  not  felt.  But  on  a  lifting  tail  machine,  sudden 
stoppage  of  the  propeller  will  relieve  the  lift  on  the  tail,  and  give  a 
tendency  to  stall  just  at  the  wrong  time,  while  sudden  starting  again 
will  nose  the  machine  over.  This  can,  of  course,  be  offset  by  having 
the  center  of  thrust  below  the  center  of  resistance.  Where  the  tail 
is  a  negative  one,  with  a  large  longitudinal  dihedral,  sudden  stoppage 
of  the  propeller  stream  causes  the  negative  tail  pressure  partly  to  be 
relieved  and  the  machine  to  nose  over  to  a  proper  gliding  angle.  And, 
when  the  propeller  is  speeded  up,  there  is  introduced  an  increased  nega- 
tive tail  pressure,  tending  to  make  the  machine  climb  at  just  the  right 
time.  On  overpowered  machines  this  tendency  of  a  negative  tail 
surface,  to  make  the  machine  climb  when  the  full  power  is  applied,  is 
an  exceedingly  comfortable  and  air- worthy  feature. 

The  most  dangerous  feature  of  a  pronounced  lifting  tail  is  in  the 
acquirement  of  higher  and  ever-increasing  speeds  on  a  steep  dive. 
The  lift  of  the  tail  is  directly  increased  as  the  square  of  the  speed,  but 
its  lever  arm  about  the  center  of  gravity  remains  the  same;  so  that, 
as  the  speed  increases  and  this  tail  lift  moment  increases,  an  unbalanced 
force  is  introduced.  The  speeds  attained  on  dives  increase  so  greatly 
and  this  tail  lift  action  may  become  so  powerful  that  the  maximum 

*  It  must  be  borne  in  mind  that,  due  to  "slip,"  the  actual  velocity  of  the  air 
thrown  back  by  the  propeller  averages  20  to  25%  faster  than  the  velocity  of  the  aero-  " 
plane. 


159 

exertion  on  the  part  of  the  pilot  on  the  elevator  control,  may  not  be 
enough  to  overcome  it.  This  exceedingly  dangerous  feature  of  the 
lifting  tail  has  resulted  in  some  very  severe  accidents. 

It  is  seen,  then,  that  a  longitudinal  dihedral  giving  the  "converg- 
ent tandem"  system,  favorable  to  inherent  stability,  is  far  preferable 
to  a  lifting  tail,  for  safety,  stability  and  airworthiness.  Their  compari- 
son on  a  basis  of  efficiency  is  not  favorable  to  the  negative  tail,  because 
the  machine  must  constantly  carry  double  the  negative  air  load,  and 
extra  resistance,  whereas  a  large  lifting  tail  will  add  just  that  much 
area  for  the  load  lifting  capacity  and  give  very  great  improvement 
in  Climbing  Rate,  Speed,  Range,  etc. 

At  times  it  is  necessary  to  compromise  stability  and  safety  for 
efficiency,  and  for  special  performances  in  the  hands  of  an  expert  a 
powerful  lift  on  the  tail  is  often  used.  Rarely,  however,  does  this  ex- 
ceed 50  to  60  pounds. 

The  effect  of  having  the  Center  of  Thrust  below  the  c.  r.  and  the 

c.  g.,  is  to  introduce  a  tendency  for  the  machine  to  assume  a  glide  angle 
when  the  engine  is  shut  off,  and  to  cl  mb  when  the  power  is  applied  — 
characteristics  that  are  certainly  more  desirable  than  a  high  thrust, 
which,  when  the  power  is  shut  off,  would  tend  to  stall  the  machine. 

The  control  of  longitudinal  balance  and  the  natural  tendency  of 
machines  to  keep  an  even  keel,  fore  and  aft,  having  been  considered, 
we  may  proceed  with  a  study  of 

ROLLING  AND   LATERAL   BALANCE. 

The  lateral  balance  of  an  aeroplane  is  understood  to  refer  to  the 
balance  of  the  wings  transversely  across  the  flight  path.  And  rolling 
is  the  movement  about  the  longitudinal  axis,  caused  by  alterations 
in  lateral  balance  in  distinction  to  pitching,  which  is  the  movement 
along  the  longitudinal  axis. 

The  lateral  balance  of  an  aeroplane  may  be  varied  by  air  disturb- 
ances and  by  the  torque  of  the  propeller  (assuming  that  the  wing  setting 
and  weight  are  symmetrical). 

The  Torque  of  the  Propeller,  is  an  air  force  due  to  the  pressure 
of  the  propeller  blades  on  the  air,  which  on  single  propeller  machines 
must  be  resisted  or  else  the  propeller  might  stand  still  and  the  motor 
turn  about  it.  The  tendency  of  the  machine  is  to  turn  'opposite  to 
the  propeller,  so  that  the  effect  of  the  torque  is  to  unbalance  the  aero- 
plane laterally,  —  in  so  much  as  it  is  necessary  to  introduce  a  lift  on 
one  side  by  a  slight  increase  in  the  incidence,  which  will  have  a  tend- 
ency to  make  the  machine  roll  in  the  same  direction  as  the  propeller 


160 

turns.  Of  course,  where  two  propellers  are  used,  working  in  opposite 
directions,  the  torque  is  neutralized.  When  the  engine  is  suddenly 
turned  on  or  off,  on  single  propeller  aeroplanes  of  high  power  and  small 
surface,  the  torque  is  a  very  perceptible  force.  It  is  interesting  to 
note  that  the  torque  of  small,  high-speed  propellers  is  very  much  less 
than  that  of  large,  slow,  geared-down  propellers. 

The  effect  of  air  disturbances  on  lateral  balance  is  merely  to  tip 
up  one  side  or  the  other,  or  to  throw  the  entire  machine  sideways, 
thereby  affecting  its  transverse  attitude. 

Since  the  actual  attitude  of  the  aeroplane  to  the  air  that  is  pass- 
ing it,  governs  the  stability  characteristics,  it  follows  that  we  are  con- 
cerned here  with  the  effect  on  the  wings  of  a  sidewise  flow  of  air,  and 
of  a  difference  in  the  angle  of  attack  on  either  side.  The  latter,  on  any 
type  of  aeroplane,  merely  makes  the  air  force  on  one  side  greater  than 
on  the  other,  and  for  the  preservation  of  the  balance  requires  a  correc- 
tive effort. 


Lateral   Stability   and   Instability. 

Pitching  requires  control  for  the  attainment  of  different  angles 
of  incidence  and  altitudes.  Yawing  requires  control  for  the  steering 
of  the  machine.  But,  independent  of  the  necessary  feature  of  bank- 
ing on  turns,  the  lateral  control  of  an  aeroplane  is  primarily  for  the  pre- 
servation of  lateral  balance. 

"Lateral  stability"  may  be  defined  as  a  natural  tendency  for  an 
aeroplane  to  keep  an  even  keel  transversely.  If  a  machine  departs 
from  an  even  keel  laterally,  it  may  roll  over  and  fall  sideways,  and 
it  is  well  for  any  pilot  to  realize,  that  of  all  conditions  of  instability, 
lateral  instability  is  the  easiest  to  acquire  and  the  most  difficult  to 
eliminate,  without  sacrificing  controllability. 

The  lateral  stability  characteristics  of  an  aeroplane  are  consid- 
ered before  taking  up  the  study  of  lateral  controls,  so  as  to  acquire  a 
better  understanding  of  their  function. 

The  effect  of  side  winds,  or,  what  amounts  to  the  same  thing,  a 
sidewise  movement  of  the  machine,  is  not  necessarily  destructive  of 
lateral  balance,  as  will  be  explained  presently. 

On  the  older  type  of  open-bodied  aeroplanes,,  with  the  wings  straight 
across  the  span,  and  at  constant  incidence,  a  side  wind  would  pass 
thru  the  machine  with  very  little  effect  in  tipping  up  one  side  more 
than  another.  But  as  soon  as  a  large  covered  fuselage  or  nacelle  is 
used,  it  is  obvious  that  a  side  wind  on  the  body  will  blanket  the  wing 
away  from  the  wind,  to  a  certain  extent,  so  that  the  machine  will  have 


161 

a  slight  tendency  to  lift  up  on  the  inside  wing.  This,  however,  is  largely 
overcome  by  the  effect  of  the  body  wheels,  etc.,  which  as  covered  areas 
below  the  c.  g.,  catch  the  side  wind  and  tend  to  turn  the  inside  wing 
down.  This  opposition  may  be  balanced  on  a  machine  quite  readily 
and  neutral  lateral  stability  obtained,  to  the  degree  that  the  machine 
will  not  tip  up  sideways.  The  entire  machine,  however,  being  acted 
upon  by  a  sideways  flow  of  air  of  less  velocity  fore  and  aft,  has  less 
lift  and  would  tend  to  stall,  were  it  not  that  the  "weathercock"  action, 
considered  later,  turns  it  to  meet  the  side  wind.  The  "side  wind" 
referred  to  is  not  of  "puff"  nature  giving  an  actual  incidence  difference 
on  the  wings  and  tipping  up  the  side  with  the  greater  angle.  This  must 
be  borne  in  mind. 

It  is  well  to  realize,  at  once,  that  any  arrangement  for  natural 
corrective  effort  when  the  machine  moves  sideways,  relative  to  the  air, 
makes  the  same  machine  roll  when  hit  by  a  side  wind. 

There  are  three  general  ways  of  obtaining  natural  lateral  sta- 
bility: 

1.  By  a  Dihedral  Angle  to  the  Span. 

The  wings  are  bent  up,  as  indicated  on  the  diagram,  and  when 
the  machine,  due  to  some  disturbance,  rolls  over,  the  low  wing  lifts 
more  than  the  high  wing  and  tends  to  correct  the  roll.  When  the 
machine  moves  sideways  the  dihedral  angle  of  the  wings  causes  a  greater 
area  and  angle  to  be  presented  to  the  air  on  the  leading  wing,  thus 
lifting  it  up.  At  the  same  time,  however,  the  higher  resistance  on 
this  wing  tends  to  make  the  machine  turn  into  the  relative  wind.  A 
side  puff  will  lift  up  the  inside  wing  that  it  first  attacks  and  then  throw 
the  machine  sideways  —  after  which  the  dihedral  causes  a  greater 
lift  on  the  low  wing,  tending  to  bring  the  machine  back  to  an  even 
keel.  This  answer  to  a  side  puff,  followed  by  the  righting  effect,  is 
always  characteristic  of  a  dihedral  wing,  and  is  uncomfortable. 

2.  By  a  Retreating  Wing  Shape. 

The  shape  of  wing  in  the  form  of  a  retreat,  as  indicated,  gives 
clearly  a  difference  in  projected  entering  edge,  and  shape  of  wing,  which, 
without  quite  as  much  sensitiveness  to  sharp  side  puffs,  at  the  same 
time  gives  considerable  difference  in  lift,  and  strong  recovery.  Like 
the  dihedral,  however,  the  difference  in  wing,  laterally,  causes  a  dif- 
ference in  resistance,  tending  to  turn  the  machine  into  the  side  wind, 
and  the  great  leverage  of  the  difference  in  lift  and  resistance  about  the 
c.  g.  makes  both  systems  exceedingly  sensitive. 

3.  By  the  Double  "High  Fin"  System. 

As  indicated  (diagram  p.  152),  the  rudder  is  placed  high  and  a  fin 
above  the  c.  g.  is  placed  forward.  The  action  of  a  side  wind  on  this 
system  tends  to  roll  the  machine  up  on  the  inside  wing,  but  while  the 


162 

dihedral  and  retreat  are  exceedingly  sensitive  to  the  least  sideways 
deviation  of  the  air  flow  from  its  direction  along  the  axis  of  the  ma- 
chine, fins  of  this  class  require  a  most  pronounced  sideways  attack  of 
the  air  before  any  considerable  effect  is  created.  Ordinary  deviations 
of  the  wind  direction  in  flight  (which  would  cause  a  dihedral  or  retreat 
to  roll  the  machine)  have  very  little  effect  on  this  fin  system,  and  the 
small  leverage  of  the  fin  pressures  about  the  c.  g.  rob  them  of  sen- 
sitiveness. At  the  same  time,  when  the  machine  itself  moves  sideways, 
to  any  great  extent,  the  high  fin  action  resists  the  movement  and  tends 
to  bank  the  machine  up  properly,  and  to  overcome  lateral  instability. 

If  the  fin  surfaces  were  below  the  c.  g.,  or  if  the  angle  across  the 
span  is  made  catedral  (turned  down)  instead  of  dihedral,  a  side  puff 
would  press  down  the  inside  wing,  and  a  side  movement  of  the  ma- 
chine would  introduce  a  force  tending  to  roll  the  machine  over,  and  to 
upset  it,  i.  e.,  lateral  instability. 

It  is  clear,  then,  that  on  a  machine  with  provision  for  corrective 
effort,  tending  to  right  the  machine  laterally  when  it  is  thrown  over 
sideways,  it  is  actually  necessary  for  the  machine  to  be  disturbed  and 
moved  sideways  before  this  corrective  force  is  created.  Every  inherent 
lateral  stability  feature,  as  a  corollary,  has  more  or  less  tendency  first 
to  permit  air  disturbances  to  roll  the  machine  —  high  fins  less  so  than 
any  other  system. 

The  position  of  the  c.  g.  may  effect  this,  in  so  far  as  a  low  c.  g. 
does  tend  to  give  a  lateral  righting  effect,  although  the  machine  is  apt 
to  swing  in  increasing  amplitude  if  too  low,  while  a  high  c.  g.,  if  above 
the  center  of  support  and  displaced,  would  tend  to  roll  the  machine 
over  and  upset  it.  The  lateral  moment  of  inertia  is  ordinarily  small, 
since  the  weights  are  practically  at  the  same  height,  laterally.  But 
on  the  old  Wright  aeroplanes,  and  the  Curtiss  flying  boats  (with  motor 
high  and  hull  low),  there  is  a  considerably  greater  inertia  laterally, 
which  makes  the  roll  slower,  and  the  resistance  to  initial  movement 
by  air  puffs  greater.  However,  this  feature  causes  the  machine,  after 
it  has  acquired  a  roll,  to  keep  on  rolling  with  considerable  force,  which 
is  detrimental  to  controllability. 

Lateral  Controls 

For  the  purposes  of  assuming  the  proper  banking  on  turns  and 
the  preservation  of  equilibrium,  laterally,  aeroplanes  are  provided 
with  transverse  controlling  devices. 

Practically  all  of  these  take  the  form  of  adjustable  surfaces  out 
at  the  sides,  in  which  changes  of  incidence  or  changes  in  camber  (as 
in  wing  flaps),  are  relied  upon  to  give  a  greater  lift  on  one  side  than  on 
the  other,  thereby  rolling  the  machine. 


163 

The  several  different  arrangements  for  lateral  control  —  warping 
of  the  wings,  ailerons  and  wing  flaps  —  have  been  explained  in  Chap. 
II.  Other  devices  for  this  purpose,  such  as  variable  surface  area  and  a 
movable  center  of  gravity,  have  been  proposed  and  tried,  but  not  as 
yet  with  any  degree  of  success. 

Because  of  extensive  patent  litigation,  great  stress  has  too  often 
been  laid  on  a  relatively  unimportant  point,  i.  e.,  the  difference  in 
the  air  resistance  of  either  side,  due  to  the  operation  of  the  transverse 
control.  If  a  wing  is  warped  to  a  greater  angle  of  incidence  on  -one 
side  and  a  lesser  angle  of  incidence  on  the  other,  and  if  the  Drift  of 
the  higher  angle  is  greater  than  the  Drift  of  the  lower  angle,  obviously 
the  machine  will  tend  to  turn  about  the  wing  with  the  greater  angle. 
The  relative  nature  of  this  difference,  however,  depends  on  the  L/D 
characteristics  of  the  particular  surface  section  used.  The  old  circular 
arc  sections  normally,  at  an  incidence  of  5°  or  6°,  had  this  characteristic. 
But  the  placid  assumption  that  all  wings  when  warped  must  necessarily 
have  a  higher  resistance  on  the  side  with  the  greater  incidence,  needs 
but  a  little  intelligent  investigation  to  be  amply  discredited  and  is 
fully  refuted  by  actual  flying  experiments.  For  example,  referring  to 
Chap.  VII,  the  Eiffel  13  bis  section  may  be  taken  as  an  illustration. 
If  flying  normally  at  an  angle  of  incidence  of  2  3^°,  the  wings  are  warped 
to  incidences  of  0°  at  one  side  and  5°  at  the  other,  it  is  seen  that  KL 
will  be  .0006  for  0°  and  .00175  for  5°,  and  that  L/D  will  be  5  at  0°  and 
15  at  5°.  Since  the  surface  area  and  speed  may  be  taken  as  the 
same  (the  machine  flying  normal),  it  follows  that  this  mean  warp, 
applied  to  the  wing  will  cause  the  side  with  the  lesser  angle  to  have 
the  higher  resistance.  The  values  of  KL  for  the  two  sides  will  deter- 
mine the  difference  in  Lift,  that  will  result  in  rolling  and  the  values 
of  L/D  will  determine  the  resistance.  The  ratio,  then,  of  KL  -*•  L/D, 
will  give  (in  the  form  of  KD)  the  actual  numerical  proportion  of  the 
resistances.  For  0°,  KL  +-  L/D  =  .0006/5  =  .000120,  and  for  5°, 
the  same  quantity  =  .00175/15  =  .000117,  which  means  that  the  wing 
with  0°  incidence  has  the  greater  resistance.  R.  A.  F.  6  section  in 
the  form  of  a  biplane  warped  3°  either  way  for  an  incidence  of  3°  (which 
would  be  an  excellent  one  to  use),  would  also  exhibit  a  higher  resist- 
ance for  the  lower  angle.  Various  angle  combinations,  on  different 
sections,  exhibit  every  shade  of  increase  and  decrease  of  the  resist- 
ance of  one  side  over  the  other,  and  in  the  tuning  up  of  a  machine  with 
warping  wings,  it  is  readily  possible  to  adjust  the  amount  of  warp  and 
washout,  so  as  largely  to  eliminate  this  characteristic  of  turning,  when 
the  wings  are  warped. 

In  sharp  turns,  the  difference  in  the  higher  speed  of  the  outside 
wing  and  slower  speed  of  the  inside  wing,  must  also  be  considered  in 
determining  which  wing  has  the  greater  Drift.  But  even  in  this  case 
it  is  possible  to  have  KL  div.  L/D  low  enough,  on  the  high  wing,  to 
make  its  resistance  equal  to  or  less  than  the  lower  wing. 


164 

Whether  or  not  ailerons  can  be  made  to  function  without  show- 
ing tendencies  to  turn  the  machine,  depends  so  much  on  their  shape, 
setting  and  interference  with  the  flow  on  the  main  surfaces,  that  it  is 
necessary  to  analyze  particular  cases.  As  a  general  rule,  they  always 
exhibit  turning  tendencies,  due  to  "choking"  effects. 

With  wing  flaps,  however,  the  combination  of  change  of  camber 
and  angle  at  the  same  time,  gives  splendid  latitude  for  proportioning 
the  transverse  control,  so  as  to  eliminate  any  tendency  to  turn  the 
machine.  Particularly  is  this  true  where  very  large  flaps  on  a  flat 
section  are  used,  which,  because  of  their  ample  size,  may  be  operated 
thru  a  small  range.  In  the  consideration  of  modern  aeroplanes  any 
very  pronounced  tendency  to  turn,  when  the  lateral  control  alone  is 
operated,  is  considered  as  evidence  of  poor  balance  and  careless  de- 
sign and  adjustment.  It  is  high  time  to  explode  the  absurd  conten- 
tion of  the  necessity  of  always  having  to  overcome  a  tendency  to  turn, 
when  the  lateral  control  is  operated,  although  this  uncomfortable 
characteristic  is  still  found  on  some  types  of  aeroplanes. 

A  change  in  Lift  on  either  side,  then,  is  made  use  of  to  control 
the  lateral  equilibrium  of  the  machine,  in  those  instances  where  the 
inherent  features  on  an  aeroplane  do  not  give  the  required  response. 
It  is  important  to  note  here,  that  the  inherent  features  of  lateral  sta- 
bility are  steadily  receiving  attention  and  development,  and  it  may 
well  be  possible,  in  view  of  the  great  progress  already  made,  that  the 
lateral  balancing  by  manual  control  will  give  way  to  an  automatic 
functioning  of  the  aeroplane  itself,  thus  eliminating  one  of  the  con- 
trols, and  rendering  flying  that  much  easier.  At  any  rate,  the  assist- 
ance to  lateral  balancing  given  by  natural  stability  features,  at  pres- 
ent, is  very  great  and  very  promising. 

TURNING. 

In  several  instances  reference  has  been  made  to  the  necessity  of 
banking  up  an  aeroplane,  so  as  to  obtain  a  centripetal  force  sufficient 
to  hold  the  aeroplane  to  the  degree  of  turn  dictated  by  the  amount  of 
rudder  movement  given.  The  manner  in  which  the  added  pressure  on 
the  wing  is  resolved  into  this  banking  force,  and  the  weight,  is  shown 
on  the  diagram,  and  for  very  steep  banks  the  magnitude  that  this 
pressure  must  attain  in  order  to  have  a  component  equal  to  the  weight 
is  evident. 

The  steering  by  rudder  is  simple  enough;  and  wide  turns  may  be 
made,  in  calm  weather,  without  any  appreciable  degree  of  bank,  but 
for  maneuvering  of  any  consequence  there  is  but  one  proper  bank 
for  any  particular  turn,  and  that  is  the  one  that  will  give  just  the  proper 
centripetal  force  to  keep  the  machine  flying  on  the  turn  at  the  same 
angle  of  incidence  relative  to  the  air,  without  any  gain  or  loss  of  altitude. 


165 

The  faster  the  speed  and  the  greater  the  weight  the  steeper  must  be  the 
bank  for  any  turn.  And  fast,  small-surfaced  machines  are  limited  in 
the  sharpness  of  turns  that  can  be  made,  since  the  centrifugal  force 
may  exceed  the  maximum  pressure  the  wings  can  give  at  the  aeroplane's 
speed,  with  the  result  that  the  machine  will  slip  outwards,  and  in  doing 
this  the  aeroplane  may  perform  the  odd  maneuver  of  sliding  outwards 
uphill  —  the  path  of  least  resistance. 

Skidding. 

If  the  bank  assumed  by  an  aeroplane  is  not  sufficient  to  hold  it 
to  a  given  turn,  the  centrifugal  force  generated  by  the  turn  will  cause 
the  machine  to  skid  outwards,  and  in  doing  so  the.  relative  flow  of  air 
past  the  machine  changes  from  axial  to  more  or  less  sideways.  A  fairly 
sharp  turn,  in  which  the  tail  was  whipped  around  by  the  rudder  with- 
out enough  bank,  would  find  the  machine  facing  around  after  completing 
the  turn,  but  with  its  speed  so  greatly  reduced,  that  a  stall  —  and  a 
bad  one  —  would  be  apt  to  follow.  In  riding  with  a  pilot  who  skids  badly 
on  his  turns,  the  side  wind  created  by  the  skidding  outward  of  the 
machine  is  readily  detected,  and  the  feeling  is  distinctly  uncomfortable. 
The  relative  side  wind  created  will  give  a  powerful  corrective  effort 
tending  to  bank  the  machine  more  steeply,  if  a  dihedral,  retreat,  or 
high  fin,  are  incorporated.  Here  is  one  of  the  important  stability  char- 
acteristics of  these  features. 

Even  though  some  pilots  of  long  experience  skid  their  turns  badly, 
the  fact  that  so  many  serious  accidents  have  resulted  directly  from 
stalling  after  a  skidded  turn  can  but  lead  to  the  conclusion  that  the 
practice  is  distinctly  inadvisable,  excepting  under  some  very  excep- 
tional landing  conditions  where  the  pilot  desires  to  "kill"  his  speed. 

Side-Slipping. 

Too  much  bank  for  a  given  turn  causes  the  machine  to  roll  over 
into  the  turn  and  to  slip  down  sideways.  This  error  ordinarily  re- 
sults in  a  nose  dive,  which,  after  a  long  fall  may,  on  a  well-balanced 
machine,  permit  of  recovery.  A  bad  side  slip,  however,  is  as  serious 
and  positive  a  destruction  of  the  equilibrium  of  the  machine,  as  is 
possible  in  ordinary  flying,  and  certainly  a  tendency  on  the  part  of 
a  pilot  to  skid  his  turns  is  far  preferable  to  overbanking  them.  Side- 
slipping also  introduces  a  sidewise  flow  of  air,  and,  consequently,  the 
inherent  stability  characteristics  obtained  from  a  dihedral,  a  retreat, 
or  a  high  fin  system,  tend  at  first  to  stiffen  a  machine  against  slipping 
and  then  to  exert  a  positive  corrective  effort.  It  is  safe  to  say  that 
in  their  recovery  power  on  this  characteristic  alone  these  features, 
particularly  a  high  fin  system,  are  distinctly  desirable  and  fully  justified. 

The  proper  combination  of  bank  and  rudder  for  any  particular 
machine,  and  skill  in  the  detection  of  skidding  or  slipping,  are  drilled 
into  the  pilot  by  instruction  in  flying  on  the  field.  But  the  following 
general  principles  may  be  stated : 


166 

Skidding  is  apt  to  result  in  a  stall,  and  is  overcome  by  decreas- 
ing the  rudder,  or  increasing  the  bank. 

Side-slipping  is  apt  to  result  in  a  nose  dive,  and  is  first  overcome 
by  more  rudder  and  less  bank,  and  later,  if  too  far  gone,  by  ruddering 
outwards. 

These  features,  relative  to  turns,  however,  are  subject  to  modi- 
fication, because  on  steep  banks  and  turns  there  is 

The  Inversion  of  the  Rudder  and  Elevator. 

The  degree  in  which  this  is  accentuated  varies  greatly  for  differ- 
ent machines  and  steepness  of  banks.  But,  as  a  general  rule,  a  turn 
banked  to  over  45°  has  begun  to  make  the  rudder  perform  the  func- 
tion of  the  elevator,  and  if  left,  offset  for  the  turn,  the  machine  will 
begin  to  spiral  down.  Whereas,  on  a  steep  bank  the  elevator  becomes 
the  rudder,  and  to  keep  the  degree  of  turn,  whether  to  right  or  left, 
the  elevator  must  be  pulled  in. 

The  "Feel"  of  a  Proper  Turn. 

Whether  to  the  pilot  or  to  the  passenger  who,  by  experience,  has 
acquired  sensitiveness  to  the  movement  of  the  aeroplane  in  the  air, 
a  properly  made  turn,  should  give  rise  to  no  change  in  the  relative 
wind,  no  tendency  of  the  body  to  swing  either  out  or  in,  but  only  to 
a  slight  increased  pressure  on  the  seat. 

Yawing  and  Directional  Stability. 

There  remains  to  be  considered  the  stability  of  direction,  or  "yaw- 
ing." If  the  directional  center  were  in  front  of  the  c.  g.,  a  side  wind 
would  obviously  tend  to  turn  the  machine  away  from  the  wind  and 
either  stall  or  upset  it  laterally.  Some  tendency  to  head  into  the  rela- 
tive wind  is  necessary.  This  is  obtained  by  having  enough  rudder 
or  fin  surface  aft  to  bring  the  directional  center  back  of  the  c.  g.  and 
is  called  "weathercock"  stability. 

However,  if  this  feature  is  accentuated  too  much,  the  machine 
tends  to  yaw  uncomfortably,  on  meeting  the  least  side  wind.  What 
is  called  "spiral  instability"  may  also  be  developed,  i.  e.,  the  machine, 
when  making  a  spiral  turn  downwards,  has  a  tendency  to  sharpen 
the  spiral  and  dive,  due-  to  the  side  pressure  on  the  body,  and  when 
spiralling  upwards  on  a  climb,  a  tendency  to  stall  is  readily  developed. 
In  this  connection  modern  fuselage  tractors  should  prove  more  dif- 
ficult to  get  out  of  a  small  field  by  a  spiral  climb  than  the  old  open- 
bodied  pushers. 

It  is  important  to  point  out  that  struts  of  large  fineness  ratio,  and 
propellers,  present  considerable  side  surface  and  affect  the  directional 
center,  at  different  angles  of  yaw,  by  the  amount  indicated  for  any 
machine  on  its  yawing  moment  diagram. 


167 

The    Dunne. 

An  examination  of  the  photographs  of  this  type  (p.  23)  reveals  an 
aeroplane  with  a  very  accentuated  retreat,  with  the  angle  of  incidence 
varying  from  positive  at  the  nose  to  negative  at  the  tips,  and  con- 
trolled solely  by  flaps  on  the  ends  of  the  wings.  While  there  is  no 
tail,  there  are  virtually  what  amounts  to  two  tails  on  this  type,  and 
the  operation  of  pitching  consists  of  turning  all  flaps  up  or  down  for 
rising  or  descending.  There  is  the  added  feature  of  the  large  braced 
panel,  on  either  end  of  the  wing  span.  The  "bustle"  and  change  in 
camber  are  not  considered  vital. 

Studying  this  type  of  machine,  it  becomes  apparent  that  the  change 
in  angle  of  incidence  gives  the  effect  of  the  "convergent  tandem"  sur- 
face arrangement,  but  with  an  exceedingly  powerful  negative  tail. 
For  a  normal  flap  setting  there  is  no  question  but  that  stalling  or  diving 
are  rendered  practically  impossible  by  this  inherent  stability  feature. 
This  might  lead  to  the  conclusion  that  the  machine  was,  in  conse- 
quence, a  constant  incidence,  constant  speed  machine,  with  no  range, 
and  a  climb  obtained  solely  from  excess  propeller  push.  This,  how- 
ever, is  actually  not  the  case,  due  to  the  changes  in  trim  obtained  from 
flap  adjustment  in  flight,  and  it  is  found  that  the  speed  range,  glide  and 
climb  of  this  type  compare  favorably  with  the  more  common  types, 
excepting  in  the  loss  of  efficiency  due  to  the  negative  pressures  at  the 
tip. 

The  retreat,  combined  with  the  change  in  angle,  give  most  re- 
markable effects  on  rolling  and  yawing.  To  begin  with,  the  least 
deviation  of  the  air  is  immediately  felt,  and  the  machine  has  a  power- 
ful tendency  to  turn  into  any  side  wind,  which  results  in  a  great  deal 
of  yawing  in  flight,  although  the  action  is  slow  and  deliberate.  Yaw- 
ing and  rolling,  however,  appear  to  be  inseparably  combined.  Oper- 
ation of  the  flaps,  inversely,  will  lift  up  one  side  and  press  down  the 
other,  and  in  doing  so  the  machine  will  tend  to  sideslip  in.  This,  how- 
ever, is  met  by  the  presentation  of  the  low  inside  wing,  across  its  en- 
tire span,  to  the  relative  side  movement,  which  causes  the  low  side  to 
lift  and  turn  at  the  same  time.  In  being  thrown  over  on  one  of  its 
sides  in  this  fashion  the  inside  side-panel  of  the  machine  receives  a 
considerable  pressure,  which  tends  still  more  to  accentuate  the  turn. 
A  skid  is,  of  course,  impossible,  since  the  machine  would  turn  into  it 
and  the  negative  tips  would  keep  the  wing  from  rising.  Various  de- 
grees of  climbing  on  turns,  or  spiralling  downwards,  are  obtained  by 
pulling  up  the  flaps  on  the  low  side,  or  pulling  down  the  flaps  on  the 
high  side,  both  maneuvers  causing  the  machine  to  be  thrown  over 
on  one  wing,  in  the  first  case  at  a  high  angle  of  incidence,  and  in  the 
other  at  a  lower  one.  Any  turn  is  at  the  expense  of  a  roll,  and  any 
roll,  even  when  caused  by  a  puff,  results  in  a  turn. 

The  inherent  tendency  and  power  of  the  machine  to  hold  an  even 
keel,  with  respect  to  the  air,  is  unmistakable.  Because  of  its  constant 


168 

answering  to  air  disturbances,  however,  the  machine  is  not  comfort- 
able and  handy  in  flight. 

The  safety  features  of  its  inherent  stability  when  used  over  water, 
where  there  is  a  great  deal  of  room  for  alighting,  makes  the  Dunne  type 
of  practical  use.  But  for  land  flying,  where  operations  in  more  or 
less  restricted  places  are  necessary,  it  is  apparent  that  the  Dunne  in- 
herent stability  features  hardly  compensate  for  the  dangers  of  catching 
a  wing  or  landing  across  wind,  due  to  the  inherent  rolling  and  yawing 
movements  of  the  machine.  These,  however,  may  be  capable  of  im- 
provement, though  they  might  very  possibly  lead  to  this  type  becoming 
more  and  more  like  the  ordinary  airworthy,  controllable  type  of  "main 
surface  and  tail"  aeroplane,  so  widely  and  successfully  used. 

The   Taube 

The  outstanding  feature  of  this  type,  a  German  "pigeon"  shape 
monoplane,  is  a  retreating  wing  shape  combined  with  upturned  wing 
tips,  of  flexible  construction.  The  upturned  wing  tips,  when  warped 
for  lateral  control,  give  a  distinctly  greater  resistance  on  the  side  that 
it  is  desired  to  lower,  thus  helping  to  turn  the  machine  properly  when 
banked.  This,  combined  with  the  retreat,  does  give  a  strong,  in- 
herent stability  action,  tending  to  eliminate  side-slipping  and  skid- 
ding, very  much  as  on  the  Dunne,  but  the  Taube  has  rudders  which 
permit  of  powerful  control,  near  the  ground.  The  flexible,  upturned 
wing-tip  feature,  renders  the  c.  p.  movement  for  the  wing  favorable 
to  longitudinal  stability,  by  increased  negative  pressure  at  the  rear 
of  the  wing  when  the  incidence  is  decreased,  and  reduction  of  this 
pressure  when  it  is  increased.  In  addition,  the  flexibility  of  the  wing 
causes  the  tip  to  be  pressed  up,  thus  giving  a  righting  effect,  when  an 
upward  puff  hits  the  wing  tip,  and  vice  versa.  Since  the  inertia  of  the 
machine  resists  movement  at  first,  this  flexibility  causes  the  machine 
to  cede  to  side  puffs  without  rolling  and  yet  to  have  an  inherent  cor- 
rective action.  Any  side  wind  action  "washes  out"  the  negative  tip, 
just  enough  to  prevent  the  machine  from  swerving  into  it  too  strongly, 
and  yet  without  sacrifice  of  the  inherent  stability  features  of  the  re- 
treating wing.  The  upturned  flexible  wing  .tip,  however,  is  wasteful 
of  power,  but  developments  along  this  line  are  apparently  promising. 

Summary 

There  may  be  drawn  from  the  consideration  of  the  common  ele- 
vator rudder  and  laterally  controlled  "main  and  tail  surface"  aero- 
plane, several  interesting  conclusions  on  airworthiness. 

The  most  airworthy  combination  for  longitudinal  control  and 
stability  would  appear  to  be  a  slightly  negative  tail,  on  a  convergent 
tandem  system,  of  which  the  flaps  form  a  large  percentage  of  the  area, 
so  that  ample  control  is  obtained  with  minimum  effort  and  drag. 


169 

On  the  lateral  equilibrium,  handy  control,  wind-fighting  qualities, 
natural  stability  and  comfort,  seem  best  obtained  by  a  combination  of 
powerful  lateral  controls,  on  an  aeroplane  with  a  high  fin  system  and 
a  slight  retreat  or  dihedral.  In  a  high  fin  system  it  must  be  borne  in 
mind  that  a  dihedral  in  side  projection  is  virtually  a  fin. 

The  arching  of  the  wing  transversely  (see  p.  152),  appears  to  give 
excellent  "fin"  qualities  without  being  too  sensitive  to  rolling  in  side 
winds. 

Since  the  approach  to  the  critical  angle  and  a  stall  greatly  affect 
the  sensitiveness  of  the  lateral  control,  thus  accentuating  tendency  to 
side  slip,  a  very  powerful  control  by  large  flaps  (variable  camber)  is 
most  desirable. 

The  degree  in  which  many  qualities  of  controllability  and  inher- 
ent stability  can  be  combined  and  accentuated  are  much  more  a  mat- 
ter for  the  personal  taste  and  "feel"  of  the  pilot  than  has  been  sup- 
posed. Some  pilots  rather  prefer  a  quick  handy  machine,  while  others 
favor  a  high  degree  of  natural  tendency  to  a  level  keel,  requiring  less 
attention  and  being  less  tiring  to  operate. 

The  necessity  at  present  of  considering  the  landing  and  starting 
conditions  as  the  real  limitations  for  flying,  need  hardly  be  emphasized. 
And  the  constant  effort  of  designers  to  extend  the  speed  range,  not 
only  to  higher  speeds  but  to  slower  speeds  for  landing,  and  to  obtain 
greater  climbing  rate  for  rising  out  of  confined  areas,  must  be  accom- 
panied by  an  equally  great  effort  to  make  the  machines  handy,  quickly 
controllable,  and  devoid  of  tricks  or  whims,  in  order  to  make  operations 
under  puffy,  treacherous  conditions  as  practical  as  possible.  It  is  un- 
fortunate that,  thus  far,  every  device  for  inherent  stability  or  automatic 
mechanically  controlled  stability  lacks  the  flexibility  and  quick  power 
of  judgment  of  the  human  brain,  necessary  for  operations  in  landing 
in  difficult  places  in  a  bad  wind.  Flying  aloft  is,  after  all,  not  so  very 
difficult,  on  a  comfortable,  well-balanced  "inherently  airworthy"  ma- 
chine, but  aside  from  the  advantage  gained  in  relieving  the  pilot  of 
having  constantly  to  operate  the  controls,  all  "inherent"  or  "automatic" 
stability  features  fail  to  add  in  safety,  unless  they  first  render  safer 
the  operation  of  coming  back  to  earth.  In  this  connection  safety 
is,  perhaps,  better  served  by  a  robust  landing  gear  on  a  machine  that  is 
perfectly  controllable,  and  in  the  hands  of  a  pilot  with  good  judgment. 

A  few  notes  in  the  form  of  directions  may  prove  of  value : 

1.  If  a  machine  is  tail  heavy,  with  a  lifting  tail,  move  the  entire 
c.  g.  of  the  machine  forward.  If  tail  heavy  with  a  negative  tail,  first 
reduce  the  negative  tail  angle,  slightly. 

2.  If  a  machine  is  nose  heavy  with  a  lifting  tail,  thus  tending  to 
dive,  first  move  the  c.  g.  back  by  some  weight  in  the  rear,  and  if  the 
characteristic  is  still  exhibited,  take  the  weight  out,  and  reduce  the  angle 
of  the  tail  two  or  three  degrees. 


3.  If  there  is  a  pronounced  tendency  for  the  machine  to  yaw,  at 
the  least  puff,  and  to  want  to  dive  steeply  into  a  spiral,  there  may  be  too 
much  "weathercock"  action,  in  which  case,  either  mount  a  small  rudder, 
or  put  some  fin  surface  forward. 

4.  If  an  unbalanced  (flap  and  fin)  rudder  is  too  hard  to  operate, 
increase  the  lever  arm.     If  a  balanced  rudder  "catches"  it  is  a  sign  that 
its  hinge  is  too  far  back. 

5.  Adjustment  of  flaps  is  capable  of  giving  various  degrees  of 
sensitiveness  and  ease  of  operation,  depending  on  the  machine.     The 
best  all-around  results  are  given  by  having  the  trailing  edge  of  the  flap 
a  little  below  the  trailing  edge  of  the  plane. 

6.  Only  two  maneuvers  need  be  resorted  to,  as  tests  of  the  im- 
portant inherent  features.     When  the  aeroplane  is  flying  horizontally, 
application  of  excess  power  without  any  elevator  change,  should  cause 
the  machine  to  climb.     And  in  a  turn  with  rudder  alone,  skidding  out 
strongly,  the  machine  should  display  a  natural  tendency  to  bank. 


A   Taube   in   flight.     The   up-     Above  —  A  modern  Taube.  —  the  flexing  of  the  wing 
turned  wing  tips  are  evident.  end  is  indicated. 

Below — A   typical    modern  German  biplane— an  Avi- 
atik.     Note  the  retreating  wing. 


CHAPTER    XIII. 
THE  EYES    OF  THE  ARMY  AND  NAVY. 

A  proper  appreciation  of  military  aeroplanes,  cannot  be  had  with- 
out giving  consideration  to  the  manner  in  which  aeroplanes  may  be  used 
in  military  and  naval  operations.  But,  in  doing  so,  let  us  not  trespass 
on  the  special  studies  of  flying  officers  in  the  use  of  aeroplanes  in  strategy 
and  tactics,  further  than  to  state  that  aeroplanes  are  used, 

1.  To  see  with; 

2.  To  communicate  with; 

3.  To  attack  with. 

Superiority  in  speed,  facility  and  accuracy  of  observation,  com- 
bined with  fighting  power  to  run  the  enemy's  aeroplanes  "out  of  the 
sky,"  or  to  do  damage  to  important  points,  must  be  sought  for  in  com- 
pany with  efficiency  in  construction,  equipment,  repair  and  operation. 

The  command  of  the  sea  belongs  to  the  ship  that  can  "overtake, 
observe  the  most,  hit  the  hardest,  and  run  away"  — with  the  greatest 
reliability. 

And  the  command  of  the  air  belongs  to  the  aeroplane  that  can 
get  up  into  the  sky  the  quickest  and  observe  the  most,  with  precision 
and  ease,  and  with  sufficient  fighting  power  to  prevent  the  enemy 
from  doing  the  same  —  all  of  which  also  must  be  accomplished  with  re- 
liability and  efficiency. 

Structural  Perfection. 

For  military  purposes,  efficiency  and  reliability  in  the  structural 
features  of  the  machines  must  be  sought  in : 

1 .  The  utmost  simplicity  in  construction,  ease  of  repair  and  facility 
in  rapid  assembly. 

2.  Resistance  to  deterioration  by  weathering  and  hard  use,  min- 
imizing the  requirements  for  parking  and  overhauling. 

3.  Standardization  of  parts,  requiring  a  minimum  of  stores  and 
facilitating  interchangeability. 

There  are  many  different  types  of  metal  fittings,  wooden  parts, 
struts,  controls  and  chassis  (see  Chap.  X),  that  differ  so  slightly  from 


172 


TYPES   OF    MILITARY   AEROPLANES 

1.  The  Bleriot  Monoplane  used  by  France  earlier  in  the  war. 

2.  The  Taube  Monoplane  used  by  Germany,  at  the  start  of  the  war. 

3.  The  Aviatilc  Tractor,  a  German  high  powered  biplane. 

4.  The  B.  E.  2  British  Reconnaissance  Tractor. 

5.  The  Twin-Motored  Caudron,  used  by  the  French.     This  machine   climbs  very 
fast  but  is  not  very  speedy. 

6.  The  Vickers  Pusher  with  gun. 

7.  The  French  Nieuport  Speed  Scout  —  a  highly  successful   type,  with   excellent 
speed  and  splendid  climb. 

8.  The  Martinsyde  Biplane,  a  typical  British  speed  scout. 


173 

one  another  in  the  use  to  which  they  are  put  that  a  Flying  Corps  can 
readily  standardize  many  of  these  features  for  all  machines.  In  general, 
welded  or  brazed  fittings,  or  laminated  wooden  members,  requiring 
special  facilities  for  manufacture,  can  largely  be  eliminated,  and  aero- 
planes for  military  purposes  with  a  few  rugged,  easily  accessible  and 
repaired  parts,  are  far  more  preferable  than  aeroplanes  with  delicate 
construction  and  countless  small  parts,  clips,  pins,  bolts  and  ''gadgets," 
all  differing  from  each  other.  The  "military"  aeroplane  is  bound  to  be 
trie  one  the  construction  of  which  is  typified  by  the  feature  —  that  only 
one  size  of  bolt,  with  the  same  thread  and  nut,  is  required  for  the  entire 
structure. 

It  is  not  at  all  impossible  to  have  an  aeroplane  so  designed,  with 
solid  wire  braces  and  simple  steel  plate  fittings,  that  the  crew  of  the 
machine  need  carry  on  the  machine  in  flight  only  a  few  tools,  a  blow 
torch,  a  soldering  iron,  a  roll  of  wire,  and  a  piece  of  steel  plate,  with 
an  extra  wheel  or  two  and  a  few  wooden  members  (and  engine  spares) 
for  the  immediate  repair  of  the  machine  without  outside  assistance. 

How  impossible  this  would  be  on  some  types  of  otherwise  sat- 
isfactory aeroplanes,  is  evident  at  the  first  glance.  The  more  difficult 
an  aeroplane  is  to  repair,  and  the  more  extensive  the  expert  labor  and 
equipment  required  to  do  it,  the  less  satisfactory  is  the  machine  for 
military  work  in  the  field. 

Observation. 

Whether  in  actually  observing  the  movements  of  troops,  the  effect 
of  artillery  fire,  or  in  taking  sights  for  and  noting  the  results  obtained 
by  gun  firing  or  bomb  dropping,  the  most  important  requirement  in 
military  aeroplanes  is  that  the  field  of  view  be  as  unrestricted  as  possible. 
Obviously,  the  "pusher"  type  offers  a  better  view  and  arc  of  gun  fire  than 
does  the  "tractor,"  but  in  the  latter  type  many  modifications,  such  as 
openings  in  the  planes  near  the  body,  the  raising  of  the  wing,  as  in  the 
"parasol"  type,  and  special  posts  for  the  observer  ("prone"  below  the 
fuselage  or  above  the  wings),  are  certain  to  be  incorporated.  The 
ordinary  tractor  monoplane  is  exceedingly  difficult  to  observe  from.  In 
this  connection  the  use  of  suitable  periscopes  is  well  worth  experiment. 

The  effect  of  speeds  of  aeroplanes  in  rendering  observation  more 
difficult  is  not  of  as  much  consequence  now,  in  view  of  the  great  height 
from  which  observations  are  made. 

Although  it  generally  has  not  been  so  considered  in  the  design 
of  the  more  common  tractors,  it  is  the  writer's  opinion  that,  for  mili- 
tary purposes,  the  "eyes"  of  the  army  and  navy  should  be  made  to 
see,  and  everything  that  is  possible  should  be  done  to  extend  the  field 
of  view. 


174 


SEVERAL   MILITARY  AEROPLANES 

1.  The  Morane-Saulnier  "Parasol"  Monoplane,  a  highly  successful  French  speed 
scout,  later  copied  in  the  German  Fokker  Monoplane. 

2.  The  Albatros  —  a  long  range,  heavy  duty  German  Tractor,  which  has  proven 
to  be  an  effective  type. 

3.  The  Twin  Tractor  German  Battleplane,  with  gunners  in  center  nacelle. 

4.  The  Voisin  "avion  de  guerre,"  a  pusher  gun  carrier. 

5.  The  Bristol  Speed  Scout  used  by  the  British. 


CHAPTER    XIV. 
CONCLUSION. 

Whether  monoplanes  or  biplanes,  tractors  or  pushers,  with  rotary 
engines  or. water  cooled  engines,  the  most  suitable  aeroplanes  for  mili- 
tary purposes  will  be  the  ones  that  are  superior  in  flight  to  the  aeroplanes 
of  the  enemy.  And  this  means  that,  precisely  as  in  naval  work,  a 
"race"  is  on  between  nations  for  superiority  in  aircraft! 

In  what,  then,  may  we  find  "superiority?" 

Simplicity  of  construction  and  efficiency  in  organization  for  main- 
tenance of  the  machines  is  not  all.  More  is  required  than  numbers, 
although  a  Flying  Corps  is  not  of  much  use  without  plenty  of  spare 
machines.  Thorough  training  and  great  personal  skill,  on  the  part 
of  the  flying  officers  —  as  important  as  the  personal .  equation  in  any 
line  of  human  endeavor  —  may  still  fail  to  give  superiority,  because 
our  aeroplanes  in  flight  must  have  command  of  the  air,  which  can  be 
obtained  only  by  ability  to  start  from  and  alight  in  more  difficult  country, 
higher  climbing  rate,  greater  speed  and  radius  of  action,  better  facilities 
for  observation  and  gun  fire,  and  greater  load-lifting  capacity. 

High  speed,  so  desirable  for  operations  in  the  air,  means  a  re- 
duction in  load-lifting  capacity,  and  limitations  of  landing  and  start- 
ing, requiring  special  aerodromes.  Facilities  for  observation  and  gun 
fire  may  necessitate  sacrifice  of  flying  efficiency  and  simplicity  of  con- 
struction. Great  radius  of  action  and  climbing  speed  may  limit  the 
load  capacity,  in  bombs,  etc.  So  that  the  ingenuity  and  skill  of  the 
engineer  officers  of  a  Flying  Corps,  must  be  exerted  to  the  utmost 
in  compromising  properly  these  opposing  features. 

It  is  barely  possible  that  there  will  be  many  types  of  military 
aeroplanes,  light,  fast  speed  scouts,  slower  load-carrying,  gun  and 
bomb  machines,  aeroplanes  especially  adapted  to  artillery  observa- 
tion, to  naval  coast  defense  work,  to  messenger  service  —  but  the  fact 
remains,  that  from  all  of  them  the  maximum  possible  view  must  be 
obtained,  with  fighting  quality  superior  to  the  enemy's  and  with  the 
greatest  load-lifting  capacity  and  climbing  speed  possible.  Every- 
thing must  be  done,  therefore,  to  improve  the  aeroplane's  efficiency 
for  military  work,  in  extending  the  speed  range,  the  climbing  rate  and 
the  load  capacity. 


176 

Instruments. 

Although  flying  is  properly  taught  on  a  basis  of  acquiring  the 
"feel"  of  the  air,  any  instrument  of  assistance  to  flying  without  adding 
considerable  weight  is  most  desirable.  On  the  dashboard  of  a  well- 
equipped  aeroplane  there  are  found  the  usual  clock,  aneroid,  fuel  gauges, 
and  engine  tachometer.  But,  in  addition  to  these,  other  devices  are 
mounted  to  indicate  the  relation  of  the  aeroplane  to  the  air.  For 
this  purpose  pitot  tube  or  pressure  plate  air  speed  indicators  are  used. 
Angles  of  incidence  to  the  air  may  be  indicated  by  a  vane  floating  in 
the  stream,  operating  a  needle  on  a  dial.  The  inclination  of  the  aero- 
plane to  the  ground  may  be  indicated  by  inclinometers,  such  as  a  bubble 
in  a  curved  tube,  or  a  pendulum.  Various  simple  devices,  such  as 
strings  or  light  vanes,  may  be  used  to  indicate  any  sidewise  movement 
or  skidding  of  the  aeroplane  thru  the  air.  In  the  determination  of 
the  speed,  climb,  etc.,  for  any  position,  the  pilot,  having  at  hand  a 
power  chart  of  the  machine,  may  read  the  r.  p.  m.  of  his  engine,  thus 
establishing  its  power;  by  reading  the  air  speed  or  the  angle  of  inci- 
dence (either  one  determines  the  other)  he  readily  notes  the  power 
required  —  so  that  he  can  judge  what  his  climbing  power  and  rate 
are,  and  what  the  fuel  consumption  is.  Or,  if  he  is  flying  on  the  hori- 
zontal and  desires  to  use  the  minimum  of  fuel  per  mile,  he  throttles  to 
the  r.  p.  m.  indicated,  and  checks  the  speed  of  greatest  economy,  by 
reading  his  angle  of  incidence  and  referring  to  his  power  chart.  A  very 
extensive  use  of  these  charts  may  be  made  in  flight,  the  only  two  instru- 
ments necessary  being  the  engine  tachometer  and  an  angle  of  incidence 
indicator.  Comparison  of  the  inclinometer  and  incidence  dial  will 
readily  reveal  whether  or  not  he  is  flying  in  up  or  down  trends,  since 
the  one  reads  the  "air  angle"  and  the  other  the  "ground  angle." 

Stabilizers   or  Automatic   Pilots. 

In  addition  to  giving  the  pilot  information  on  his  flying,  there  are 
the  "automatic  stabilizers,"  — instruments  to  relieve  him  of  having  to 
hold  the  controls.  Inherent  features  of  airworthiness  in  the  machines 
will  also  do  this,  but  only  after  answering  to  disturbances  in  much 
greater  measure  than  a  delicately  adjusted  stabilizer.  The  latter,  also, 
if  pendulum  of  gyroscope  governed,  holds  the  aeroplane  to  a  "base  line" 
relative  to  the  ground  and  not  to  the  air. 

Level  flight  is  thus  obtained,  with  more  or  less  success,  and  with 
pendulum  and  gyroscope  stabilizers  it  is  possible  for  the  pilot  to  be  re- 
lieved of  having  to  attend  to  the  controls,  in  that  the  "stabilizer"  or 
"automatic  pilot"  keeps  the  aeroplane  on  a  fixed  and  steady  course. 
This  requires  careful  adjustment  for  each  particular  type  of  aeroplane, 
however,  and  since  flying  on  an  airworthy  machine,  with  inherent 
features  not  too  much  accentuated  is  comfortably  possible  with  con- 
trols locked,  reasons  of  safety  alone  do  not  demand  "automatic 
stabilizers,"  in  view  of  their  added  complication. 


177 

Stabilizers  can  also  be  made  to  bank  an  aeroplane  properly  on  a 
turn  and  hold  it,  with  an  accuracy  and  precision  that  is  remarkable. 

For  night  flying,  an  automatic  pilot  mechanism  has  very  great 
advantages.  And  for  bomb  dropping,  etc.,  in  improving  the  steadiness 
of  the  aeroplane  as  a  platform,  it  is  a  valuable  auxiliary. 

Performances   and   Operation. 

It  is  of  the  utmost  importance  in  military  operations  to  have  in- 
formation on  the  radius  of  action,  the  load-lifting  capacity  and  the 
speeds  of  the  aeroplanes  to  be  used.  For  the  purpose  of  assisting  in 
these  matters,  particular  attention  has  been  given  to  the  prediction  of 
the  performances  of  aeroplanes. 

In  choosing  machines  to  lift  a  great  load  of  bombs  here,  or  to  travel 
a  great  distance  at  high  speed  on  a  raid  there,  or  to  climb  up  very  quickly 
and  return  with  information  for  some  other  purpose,  a  study  of  the 
Power  Charts  and  data  on  fuel  consumption  and  lifting  capacity  (Chap. 
VII  and  VIII)  is  not  merely  helpful  —  it  is  necessary.  And  for  all 
intelligent  military  aviators,  a  study  of  this  kind  is  of  great  import- 
ance. In  fitting  auxiliary  devices,  guns,  bomb  droppers,  etc.,  in- 
formation on  the  resistances  (Chap.  IV),  and  on  proper  balancing 
of  the  weights  (Chap.  XII),  as  well  as  the  strength  of  parts  necessary 
to  do  the  work  desired  (Chap.  IX  and  X),  may  be  applied  directly 
to  such  problems  in  the  field. 

The  conditions  of  actual  operation  of  aeroplanes  as  dictated  by 
the  weather  are  quite  variable.  Fog  is  the  most  serious  detriment 
to  flying,  next  to  which  may  be  put  the  possible  limitations  of  start- 
ing and  alighting.  In  certain  winds  some  small  fields  are  not  difficult 
to  negotiate,  but  under  different  conditions  they  may  prove  impos- 
sible. Here  again  local  conditions  bring  up  questions  of  suitability  of 
various  aeroplanes  in  such  a  way  that  countless  problems  are  pre- 
sented requiring  "heady"  resourcefulness.  For  example,  a  condition 
may  readily  arise  where  a  machine  of  slow  speed,  which  gets  off  the 
ground  in  a  short  run  but  does  not  climb  fast,  may  be  preferable  to  a 
very  much  faster  machine  of  longer  run,  even  though  its  climb  is  better. 

Not  only  may  the  performances  of  an  aeroplane  be  studied  on 
the  field,  but  in  their  work  the  technical  officers  and  engineers  of  a 
Flying  Corps  should  be  able  to  judge  of  the  probable  performances  of 
an  aeroplane  from  charts  and  drawings  —  sufficiently  to  limit  the  ac- 
ceptance tests  to  satisfactory  construction  and  balance,  and  to  choose 
the  aeroplanes  needed  for  any  particular  purpose  before  delivery.  It 
is  decidedly  inefficient  blindly  to  try  a  machine  out  for  some  special 
performance  without  first  going  through  all  the  simple  computations 
and  determinations  bearing  thereon. 

The  operation  of  aeroplanes  in  a  wind  requires  consideration  of 
the  direction  and  force  of  the  wind,  in  determining  the  radius  of  action. 
The  aeroplane  always  keeps  its  particular  attitude  and  speed  relative 


178 


INTERESTING    WAR    LESSONS 

The  Caudron  Twin  Tractor,  with  centre  nacelle  for  gunner  (upper  left)  gained  excellent  climb, 
at  the  sacrifice  of  speed,  by  the  two-motor  arrangement.  This,  however,  obstructs  the  view  of  the  pilot. 
Below  it  is  shown  the  Curtiss  Seaplane,  with  two  tractor  motors  of  so  called  "America"  type.  This  large 
craft,  due  to  its  size  shows  good  seaworthiness,  but  at  the  expense  of  flying  characteristics.  At  lower 
left  is  shown  a  view  of  the  huge  Sikorsky  multi-motored  machine,  used  by  Russia.  In  general,  huge 
land  aeroplanes  have  not  yet  attained  the  perfection  or  excellence  in  performance  that  will  warrant  their 
adoption  as  Zeppelin  fighters,  although  large  gun  carriers  are  very  successfully  used  at  night  as  "avions 
de  bombardement." 

On  the  right  are  shown  some  speed  scouts.  The  big  aeroplane  has  still  much  to  prove  for  itself, 
although  its  gradual  development  is  inevitable.  The  light,  fast  speed  scout,  however,  is  -decidedly  the 
success  of  all  war  aeroplanes.  Operated  by  one  man  only,  who  is  expert  in  both  flying  and  military  work, 
these  small  machines  outclimb  and  outspeed  all  the  heavier,  larger  types.  Their  offensive  value  has 
consisted  merely  of  a  light  machine  gun,  shooting  over  or  through  the  propeller.  The  Nieuport,  as  seen 
from  a  companion  machine  in  flight,  is  shown  at  the  top  right.  Below  it  is  a  "pusher"  type  speed  scout, 
built  in  England,  and  at  lower  right  is  the  S.  E.  4,  a  very  fast  machine,  constructed  by  the  British  Gov- 
ernment. 

Speed  scouts  are  frequently  equipped  with  an  automatic  pilot  such  as  the  Sperry  gyroscope,  to 
relieve  the  pilot  of  having  to  operate  the  controls,  and  making  the  aeroplane  a  steadier  platform  for  gun 
fire. 

Great  excess  of  power  gives  these  small  machines  a  very  real  advantage  in  acquiring  command 
of  the  air. 


179 

to  the  body  of  air  it  is  passing  thru,  but  this  entire  body  in  the  form 
of  wind  may  be  moving  — •  so  that  the  aeroplane's  travel  relative  to  the 
earth  becomes  the  resultant  of  its  velocity  and  the  wind  velocity. 

In  naval  work,  only  speeds  on  the  horizontal  need  be  considered, 
but  in  speed  thru  the  air  an  aeroplane  must  have  superior  velocity  up- 
wards as  well  as  onwards. 

For  tactical  observation  and  for  artillery  work,  it  becomes  of  the 
utmost  importance  to  consider  that  climbing  speed,  after  all,  may  prove 
the  most,  vital  criterion  of  superiority  —  since  a  slower  machine,  superior 
in  load  capacity  and  climbing  speed,  may  dominate  a  faster  machine, 
and  climb  away  from  it  —  so  that  efficiency  may  well  be  strained  to  the 
limit  to  obtain  speed  upwards. 

The  fight  between  aeroplane  and  aeroplane  is  where  the  real  test 
of  superiority  is  certain  to  be  found,  and  both  the  moral  ascendency 
and  actual  command  of  the  air,  goes  to  the  pilot  whose  aeroplane  and 
whose  skill  permits  him  to  climb  over  and  dominate  or  drive  the  enemy 
out  of  the  sky. 

It  has  been  assumed  throughout  this  work  that  one  of  the  most 
vital  parts  of  an  aeroplane — the  motor  —  was  working  smoothly  and 
without  a  miss.  If  not  universally  the  case,  at  present,  the  day  is 
certainly  not  far  distant  when  aeroplane  motors  may  be  relied  upon 
exactly  as  are  automobile  motors  today. 

Attention  has  purposely  not  been  given  to  the  military  technique 
of  the  use  of  aeroplanes  in  military  or  naval  operations,  neither  has 
any  special  attention  been  given  to  the  art  of  flying,  cross  country 
navigation,  etc.  —  features  that  are  acquired  by  the  military  aviator 
from  the  officers  and  instructors  of  the  Flying  Corps,  in  their  routine 
work.  Consideration  has  been  given  to  the  military  aeroplane,  for  the 
particular  purpose  of  assisting  the  military  aviator  or  student  to  acquire 
a  better  appreciation  of  the  machine,  a  fuller  knowledge  of  why  it  flies 
and  what  he  may  expect  of  it,  in  performance,  in  strength  and  in  flying 
characteristics. 


INDEX 


Absolute  system  of  units,  72. 

Acceleration,  machines  of,  29. 

Aeroboats,  21,  142,  146. 

Aerodynamics,  36,  69. 

Aerofoils,  67,  73-75. 

Aeroplane,  definite,  11,  13,  90. 

Aeroplane  types,  14,  90,  152. 

Aeroplane  characteristics,  89. 

Aeroplane  linen,  136. 

Ailerons,  def.,  14. 

Air  flow,  diagrams,  68. 

Air,  nature  of,  43. 

Air  resistances,  42,  45,  140. 

Airships,  def.,  10. 

Airworthiness,  147. 

Alignment  of  wings,  122-125. 

America,  flying  boat,  22. 

Angle  ranges,  153. 

Angle  of  incidence,  57,  60. 

Angular  velocity,  31. 

Areas  of  figures,  40. 

Ash,  134,  138. 

Aspect  ratio,  44,  57,  60. 

Aspect  ratio,  corrected  table,  78. 

Aspect  ratio  of  aerofoils,  79,  80. 

Aspect  ratio  of  hydro  surfaces,  142. 

Atmosphere,  42. 

Automatic  stability,  24,  176. 

Axes  of  rotation  of  aeroplane,  13. 


Balance,  directions  for,  170. 

Balance  of  hydros,  143. 

Balanced  rudder,  78. 

Beam  sections,  114. 

Beam,  bending  moments,  114. 

Bending  moments,  general,  115,  116. 

Biplanes,  tractors  and  pushers,  16. 

Biplane  effect,  86. 

Biplane  table,  87. 

Bleriot  monoplane,  20. 

Blunt  nose  aerofoil,  76. 

Boat  upkeep,  140. 

Bolts,  locking  of,  127. 

Bolts,  strength  of,  137. 

Bracing  of  wings,  110,  111. 

Breguet  Hydro,  22. 

Burgess-Dunne,  23. 

Burgess-Loening  tractor,  146. 

Bustle,  23. 


Cables,  48,  51. 

Cable  ends,  132. 

Cable  strengths,  137. 

Cabre  attitude,  18,  152,  153. 

Camber,  57. 

Cambered  planes,  62,  63. 

Camber  of  upper  and  lower  face,  76,  78. 

Centers  of  forces,  148. 


Center  of  pressure,  60,  61,  111. 

Center  of  pressure  total  for  aeroplane,  150. 

Center  of  gravity,   by  moment   method 

148,  149. 

Centers  co-incident  in  balancing,  150. 
Center  panel,  15. 

Centrifugal  and  Centripetal  forces,  30. 
Centripetal  force,  stress,  107. 
Charts,  40. 
Chord,  60. 
Climbing  Rate,  104. 
Complements,  forces,  39. 
Composition  of  forces,  38. 
Constants,  27. 

Construction  details,  127,  130,  131. 
Controls,  14. 
Controllability,  148. 
Co-ordinates,  40. 
Crystallization,  132. 
Curtiss  Flying  Boat,  22. 
Curtiss  Tractor,  18. 
Cylinders,  resistances,  48,  51. 


Decalage,  93. 
Deflection  of  Beams,  114. 
Deflection  of  air,  theory,  71. 
Density  of  air,  35. 
Deperdussin  monoplane,  20. 
Depth  of  curvature,  65. 
Depth  of  aerofoil  section,  76. 
Diametral  plane,  49. 
Dihedral  angle,  17. 
Dihedral  arched,  152,  161. 
Dihedral,  longitudinal,  158. 
Dipping  front  edge,  74. 
Directional  stability,  166. 
Directional  center,  151,  152. 
Dirigible  Balloons,  10. 
Diving,  def.,  155. 
Diving  speed,  max.,  107. 
Dopes  for  wing  surfaces,  134. 
Drift,  56,  58,  91,  99. 
Dunne  aeroplane,  23,  167. 


Efficiency  in  power,  38. 

Eiffel's  experiments,  44,  46. 

Eiffel  wings,  Nos.  19,  37,  38,  42— p..  82. 

Eiffel  wings,  Nos.  41,  45,  59,  60— p.,  83. 

Elastic  limit,  35. 

Elasticity,  32. 

Elevator,  def.,  14. 

Elevator  flap,  descr.,  15. 

Elevator,  rear  and  front,  16. 

Elevator  Masking,  155. 

Empennages,  def.,  19. 

Empirical  constants,  27. 

Energy,  kinetic  and  potential,  36. 

Enlargement  from  model  tests,  80. 

Equilibrium  of  the  air  forces,  150. 


180 


Fairing,  44. 

Fatigue,  132. 

Fiber  stress,  117. 

Fineness  ratio,  44. 

Fins,  17. 

Fin  system,  double  high  fins,  152,  162. 

Flap  and  fin  rudder,  17,  78. 

Flaps,  def.,  14. 

Flat  planes,  59,  60. 

Flotation..  140. 

Flying  machine,  def.,  11. 

Flying  in  region  of  inverse  controls,  154. 

Flying  at  low  angles,  156. 

Flying  Boats,  21. 

Follow  thru,  119. 

Forces,  graphics  of,  34. 

Formulae,  derived  and  empirical,  26,  27. 

Formulae  for  Aerodynamics,  47. 

Friction  def.,  43. 

Fuel  Charts,  96. 

Fuselages,  56,  16,  17. 


Mechanics,  theory  of,  29. 

Metals,  weights  and  strengths,  132-138. 

Metric  system,  72. 

Metric  conversion,  40. 

Modulus  of  Elasticity,  33. 

Moment  curves  for  models,  152. 

Moments  of  forces,  39. 

Moments  of  Inertia  of  Aeroplanes,  151. 

Moments  of  Inertia,  mechanics,  30,  31. 

Monocoques,  20. 

Monoplanes,  20,  21. 


N 


Nacelle,  19. 
Newton,  46. 
Nieuport,  41. 
Normal  plane,  49. 
Normal  surface,  68. 
NPL  4  wing,  84,  85. 


Gap,  definition,  78. 

Gases,  mechanics  of,  36. 

Gliding,  103,  155. 

Goettingen  results,  52,  53. 

Graphical  stress  methods,  34,  40,  110. 

Gyroscope,  31. 


H 

Helicopter,  definition,  11. 

Horsepower,  defined,  37. 

Horsepower,  available  and  required,  111. 

Hydro-aeroplanes,  142. 

Hydroplaning,  141,  142,  143. 


I 

Inclination  of  Aeroplane,  92. 
Inclined  surfaces,  57,  68,  71. 
Inherent  stability,  23. 
Interference  of  aerofoils,  86. 
Interference  of  tail  plane,  88. 
Inversion  of  Rudder  and  Elevator,  166. 


Langley's  experiments,  46. 
Lateral  balance,  159. 
Lateral  center,  150. 
Lateral  control,  14,  163,  164. 
Lateral  stability,  160. 
Leading  edge,  57. 
Lifting  capacity,  91,  94,  96. 
Lift  and  Drift,  58. 
Lilienthal's  Tangential,  63. 
Loening  Aeroboat,  22. 
Longitudinal  stability,  156. 


M 

Marine  aeroplanes,  22,  139. 
Martin  tractor,  18. 
Master  diameter,  49. 
Mathematical  signs,  28. 


Ordinate,  maximum,  77. 
Ornithopter,  definition,  11. 
Overhang,  definition,  15. 


Pancaking,  155. 

Parallel  normal  surfaces,  51. 

Parseval  dirigible  shapes,  62. 

Pendulum,  mechanics  of,  30. 

Pfeilfliegers,  23,  170. 

Phillips  Entry,  77,  78. 

"Pique,  vol,"  152,  153. 

Pitching,  definition,  13. 

Pitching,  stability.  157. 

Pitot  Tube,  35. 

Polar  Co-ordinates,  34. 

Pontoon  bottom,  aspect  ratio,  142. 

Pontoon,  double  float  system,  142. 

Pontoons,  sections  and  details,  142. 

Power,  definition  of,  37. 

Power  charts,  96. 

Power  required  and  available,  91. 

Pressure,  definition,  43. 

Propeller,  general,  101,  102. 

Propeller  balancing,  126. 

Propeller  tipping,  126. 

Propeller  thrust,  109. 

Propeller  diagram  and  offsets,  126. 

Propeller  stream  action  in  tail,  158. 

Propeller  Torque,  159. 

Pushers,  descr.  and  def.,  14,  16,  19. 


RAF  6  wing,  84,  85. 

Raked  end  wing,  78. 

Rectangles,  resistances,  50. 

Rectangular  Co-ordinates,  34. 

Regime  of  flight,  153. 

Resistance,  Total,  to  motion,  91,  98,  99. 

Resisting  Moment  of  a  beam,  116. 

Resolution  of  Forces  on  planes,  60. 

Resultant,  38. 

Retreat,  definition,  23,  78. 

Retreat,  effects,  152,  161. 

Reversed  curve  sections,  65,  77. 


181 


Reversed  curve  wings,  84,  85. 
Rolling,  definition,  13. 
Rolling  control,  14. 
Rolling  and  stability,  159. 
Rotary  motion,  mechanics  of,  30. 
Rounded  end  plane,  78. 
Rudder,  definition,  14. 


Safety  Factors,  105. 

Seaworthiness,  140. 

Shapes  of  Pontoons,  144. 

Sheet  Metal  table,  136. 

Shoulder  yoke,  def.,  17. 

Side  Slipping,  165. 

Signal  Corps  Tractor,  18. 

Similitude,  Theory  of,  101. 

Sines  and  Cosines,  28. 

Skidding,  165. 

Span,  definition,  57. 

Spars,  stresses  in,  113. 

Spar,  sections,  114. 

Spar  weakening,  118. 

Specific  Gravity,  35. 

Speed  and  Scale  effect,  80. 

Speed,  High  and  Low,  104. 

Spheres,  Resistance  of,  48,  51. 

Spiral  Instability,  167. 

Spruce,  134,  138. 

Square  normal  plates,  48,  49. 

Stability,  definition,  147. 

Stagger,  def.,  17,  78. 

Staggering,  effect  of,  87. 

Stalling,  def.,  154. 

Steel,  general,  129. 

Steel,  tables,  136,  138. 

Strain,  def.,  33. 

Stream  lines,  43,  45,  52,  53. 

Stream  photos,  68. 

Stress,  def.,  33. 

Stresses,  general,  105. 

Stresses,  maximum,  107. 

Stresses,  on  body  joints,  119. 

Structural  Air  Resistance,  91,  98. 

Struts,  54,  55. 

Strut  Formula,  111. 

Suction  on  top  aerofoils,  74,  78. 


Tail  planes,  discussion,  85. 
Tail  skid,  17. 

Taube,  descr.,  23,  168,  170. 
Tandem   system,   convergent   and   diver- 
gent, 158. 

Tandem  seating,  16. 
Tanks,  formulae  for,  40. 
Thrust,  center  of,  102,  159. 
Torque,  31,  102. 
Tractor,  def.  and  disc.,  14,  16. 
Trailing  edge,  57. 
Triangles,  solution  of,  27,  28. 
Trueing  up  wings,  122. 


U 


Ultimate  Resistance,  35. 
Unpacking  of  aeroplanes,  121. 


"V"  bottom  on  pontoons  or  hulls,  141,  142. 
Variable  camber  wing,  84,  85. 
Velocity,  mechanics  of,  29. 
Visualizing  air,  its  importance,  44. 
Volumes  of  various  shapes,  for  tankage,  40. 


W 

Warping,  14,  77. 
Washout,  93. 

Weathercock  stability,  166. 
Wheels,  resistance  of,  55. 
Wing,  covering,  134. 
Wing,  loading,  111. 
Wing  stresses,  109. 
Wires,  48. 

Wire  tightening,  118. 
Wire  ends  for  solid  wire,  132. 
Work,  definition,  36. 
Wright,  Aeroplanes,  19. 


Yawing,  definition,  13. 
Yawing,  stability,  166. 


Tail  high  attitude,  18,  152. 
Tail  interference,  94. 


Zeppelin  dirigible  balloon,  10. 


182 


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